mobile: cache debug.keystore to avoid adb install certificate errors
mobile: cache debug.keystore to avoid adb install certificate errors

Otherwise, successive adb installs will require the existing apk to be
removed. If no android.keystore is generated during docker build, one
will be generated by the NDK toolchain during docker run.
It will end up the applications are being signed with different
certificates on each docker run.

Fixes #9498.

Change-Id: Ibac202b828224287ec6fdc3eaff659de5fce61c3
Reviewed-on: https://go-review.googlesource.com/2266
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

DanielWorld/mobile,samanalysis/mobile,b54898533/mobile,tempbottle/mobile,grategames/mobile,xing/gomobile,DanielWorld/mobile,goldcaddy77/mobile,DanielWorld/mobile,lonphy23t/mobile,lonphy23t/mobile,0x90sled/mobile,zach-klippenstein/mobile,ichu501/mobile,grategames/mobile,cainiaocome/mobile,swisspol/mobile,xing/gomobile,lonphy23t/mobile,ichu501/mobile,qskycolor/mobile,0x90sled/mobile,goldcaddy77/mobile,freeman0211/mobile,tempbottle/mobile,ppamorim/mobile,lonphy23t/mobile,ppamorim/mobile,golang/mobile,MaxCy330/mobile,lonphy23t/mobile,MaxCy330/mobile,cainiaocome/mobile,0x90sled/mobile,tempbottle/mobile,zach-klippenstein/mobile,samanalysis/mobile,golang/mobile,freeman0211/mobile,xing/gomobile,tempbottle/mobile,golang/mobile,0x90sled/mobile,steeve/mobile,cainiaocome/mobile,qskycolor/mobile,steeve/mobile,ppamorim/mobile,goldcaddy77/mobile,steeve/mobile,qskycolor/mobile,steeve/mobile,freeman0211/mobile,ichu501/mobile,tempbottle/mobile,xing/gomobile,MaxCy330/mobile,samanalysis/mobile,ppamorim/mobile,zach-klippenstein/mobile,b54898533/mobile,zach-klippenstein/mobile,cainiaocome/mobile,grategames/mobile,ichu501/mobile,swisspol/mobile,swisspol/mobile,b54898533/mobile,golang/mobile,cainiaocome/mobile,grategames/mobile,MaxCy330/mobile,freeman0211/mobile,samanalysis/mobile,DanielWorld/mobile,b54898533/mobile,0x90sled/mobile,xing/gomobile,goldcaddy77/mobile,MaxCy330/mobile,b54898533/mobile,goldcaddy77/mobile,swisspol/mobile,ichu501/mobile,swisspol/mobile,samanalysis/mobile,qskycolor/mobile,ppamorim/mobile