Improved build rule generation for Android flatbuffer headers.
Improved build rule generation for Android flatbuffer headers.

* Added the ability to create a build target for generated headers.
* Made it possible for generated header targets to depend upon each
  other or arbitrary build targets.

Tested:
Verified some pretty complex libraries with numerous flatbuffer schema
dependencies build using this macro on Linux with the NDK.
Bug: 25188384

Change-Id: I846855a50808e58c34cdf7086e93e7ea0df69e0d

chobie/flatbuffers,stewartmiles/flatbuffers,chobie/flatbuffers,wheelcomplex/flatbuffers,chobie/flatbuffers,kiyoto-suzuki/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,chenkai036/flatbuffers,rw/flatbuffers,google/flatbuffers,vitalyisaev2/flatbuffers,bog-dan-ro/flatbuffers,rw/flatbuffers,splhack/flatbuffers,endorph-soft/flatbuffers,benssson/flatbuffers,endorph-soft/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,kiyoto-suzuki/flatbuffers,wheelcomplex/flatbuffers,hgl888/flatbuffers,kadirahq/flatbuffers,chenkai036/flatbuffers,layzerar/flatbuffers,chobie/flatbuffers,stewartmiles/flatbuffers,rw/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,wheelcomplex/flatbuffers,Reimerei/flatbuffers,MiCHiLU/flatbuffers,alexames/flatbuffers,splhack/flatbuffers,rw/flatbuffers,fuchsia-mirror/third_party-flatbuffers,benssson/flatbuffers,evanw/flatbuffers,fuchsia-mirror/third_party-flatbuffers,jekinchen/flatjni,bog-dan-ro/flatbuffers,zarianw/flatbuffers,chenkai036/flatbuffers,eburkitt/flatbuffers,alexames/flatbuffers,FrankStain/flatbuffers,alexames/flatbuffers,vitalyisaev2/flatbuffers,jbrads/flatbuffers,benssson/flatbuffers,royalharsh/flatbuffers,jekinchen/flatjni,layzerar/flatbuffers,bog-dan-ro/flatbuffers,alexames/flatbuffers,splhack/flatbuffers,fuchsia-mirror/third_party-flatbuffers,endorph-soft/flatbuffers,hgl888/flatbuffers,rw/flatbuffers,eburkitt/flatbuffers,FrankStain/flatbuffers,rw/flatbuffers,google/flatbuffers,zarianw/flatbuffers,eburkitt/flatbuffers,vitalyisaev2/flatbuffers,eburkitt/flatbuffers,jekinchen/flatjni,evolutional/flatbuffers,MiCHiLU/flatbuffers,chobie/flatbuffers,fuchsia-mirror/third_party-flatbuffers,evolutional/flatbuffers,rw/flatbuffers,FrankStain/flatbuffers,google/flatbuffers,FrankStain/flatbuffers,stewartmiles/flatbuffers,jbrads/flatbuffers,google/flatbuffers,FrankStain/flatbuffers,splhack/flatbuffers,chobie/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,Reimerei/flatbuffers,jasonstubbs/flatbuffers,rw/flatbuffers,endorph-soft/flatbuffers,chenkai036/flatbuffers,endorph-soft/flatbuffers,vitalyisaev2/flatbuffers,kiyoto-suzuki/flatbuffers,evolutional/flatbuffers,jasonstubbs/flatbuffers,jekinchen/flatjni,jasonstubbs/flatbuffers,royalharsh/flatbuffers,evanw/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,chenkai036/flatbuffers,Reimerei/flatbuffers,alexames/flatbuffers,jasonstubbs/flatbuffers,zarianw/flatbuffers,vitalyisaev2/flatbuffers,kadirahq/flatbuffers,SinbadFreedom/FlatBuffer,evolutional/flatbuffers,iperry/flatbuffers,alexames/flatbuffers,endorph-soft/flatbuffers,endorph-soft/flatbuffers,MiCHiLU/flatbuffers,EricLagergren/flatbuffers,FrankStain/flatbuffers,google/flatbuffers,kadirahq/flatbuffers,Reimerei/flatbuffers,hgl888/flatbuffers,SinbadFreedom/FlatBuffer,bog-dan-ro/flatbuffers,google/flatbuffers,SinbadFreedom/FlatBuffer,MiCHiLU/flatbuffers,stewartmiles/flatbuffers,evanw/flatbuffers,Reimerei/flatbuffers,evolutional/flatbuffers,splhack/flatbuffers,MiCHiLU/flatbuffers,wheelcomplex/flatbuffers,royalharsh/flatbuffers,SinbadFreedom/FlatBuffer,EricLagergren/flatbuffers,pjulien/flatbuffers,eburkitt/flatbuffers,kadirahq/flatbuffers,chobie/flatbuffers,zarianw/flatbuffers,rw/flatbuffers,chenkai036/flatbuffers,EricLagergren/flatbuffers,vitalyisaev2/flatbuffers,EricLagergren/flatbuffers,jasonstubbs/flatbuffers,jekinchen/flatjni,layzerar/flatbuffers,chenkai036/flatbuffers,wheelcomplex/flatbuffers,bog-dan-ro/flatbuffers,MiCHiLU/flatbuffers,Reimerei/flatbuffers,rw/flatbuffers,SinbadFreedom/FlatBuffer,benssson/flatbuffers,endorph-soft/flatbuffers,alexames/flatbuffers,google/flatbuffers,vitalyisaev2/flatbuffers,jbrads/flatbuffers,iperry/flatbuffers,google/flatbuffers,SinbadFreedom/FlatBuffer,layzerar/flatbuffers,evanw/flatbuffers,jekinchen/flatjni,fuchsia-mirror/third_party-flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,jekinchen/flatjni,chobie/flatbuffers,rw/flatbuffers,google/flatbuffers,rw/flatbuffers,jekinchen/flatjni,jbrads/flatbuffers,kiyoto-suzuki/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,SinbadFreedom/FlatBuffer,iperry/flatbuffers,jbrads/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,jbrads/flatbuffers,zarianw/flatbuffers,pjulien/flatbuffers,jekinchen/flatjni,evanw/flatbuffers,evolutional/flatbuffers,vitalyisaev2/flatbuffers,pjulien/flatbuffers,bog-dan-ro/flatbuffers,MiCHiLU/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,benssson/flatbuffers,kiyoto-suzuki/flatbuffers,alexames/flatbuffers,evanw/flatbuffers,stewartmiles/flatbuffers,hgl888/flatbuffers,kiyoto-suzuki/flatbuffers,alexames/flatbuffers,endorph-soft/flatbuffers,google/flatbuffers,chenkai036/flatbuffers,evolutional/flatbuffers,benssson/flatbuffers,alexames/flatbuffers,bog-dan-ro/flatbuffers,fuchsia-mirror/third_party-flatbuffers,jbrads/flatbuffers,wheelcomplex/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,zarianw/flatbuffers,EricLagergren/flatbuffers,pjulien/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,eburkitt/flatbuffers,wheelcomplex/flatbuffers,kiyoto-suzuki/flatbuffers,jekinchen/flatjni,iperry/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,evanw/flatbuffers,splhack/flatbuffers,hgl888/flatbuffers,endorph-soft/flatbuffers,alexames/flatbuffers,splhack/flatbuffers,EricLagergren/flatbuffers,jbrads/flatbuffers,chenkai036/flatbuffers,layzerar/flatbuffers,hgl888/flatbuffers,pjulien/flatbuffers,zarianw/flatbuffers,royalharsh/flatbuffers,benssson/flatbuffers,jasonstubbs/flatbuffers,eburkitt/flatbuffers,kadirahq/flatbuffers,iperry/flatbuffers,iperry/flatbuffers,google/flatbuffers,evanw/flatbuffers,splhack/flatbuffers,DavadDi/flatbuffers,google/flatbuffers,google/flatbuffers,kiyoto-suzuki/flatbuffers,chenkai036/flatbuffers,fuchsia-mirror/third_party-flatbuffers,kadirahq/flatbuffers,iperry/flatbuffers,Reimerei/flatbuffers,FrankStain/flatbuffers,MiCHiLU/flatbuffers,iperry/flatbuffers,jasonstubbs/flatbuffers,benssson/flatbuffers,bog-dan-ro/flatbuffers,zarianw/flatbuffers,DavadDi/flatbuffers,eburkitt/flatbuffers,EricLagergren/flatbuffers,stewartmiles/flatbuffers,fuchsia-mirror/third_party-flatbuffers,kiyoto-suzuki/flatbuffers,alexames/flatbuffers,stewartmiles/flatbuffers,DavadDi/flatbuffers,royalharsh/flatbuffers,royalharsh/flatbuffers,wheelcomplex/flatbuffers,jasonstubbs/flatbuffers,kadirahq/flatbuffers,SinbadFreedom/FlatBuffer,kadirahq/flatbuffers,layzerar/flatbuffers,FrankStain/flatbuffers,layzerar/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,EricLagergren/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers