Pin FlatBuffers Android Travis builds to NDK r17c (#5460)
Pin FlatBuffers Android Travis builds to NDK r17c (#5460)

r17c is the last Android NDK to include stlport and gnustl.
We want to continue to support these deprecated STLs until we have
confidence few enough customers are using them.
stewartmiles/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,google/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,google/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,evolutional/flatbuffers,google/flatbuffers,evolutional/flatbuffers,royalharsh/flatbuffers,evolutional/flatbuffers,royalharsh/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,stewartmiles/flatbuffers,DavadDi/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,royalharsh/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,google/flatbuffers,evolutional/flatbuffers,royalharsh/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,pjulien/flatbuffers