Add AaptBuild task

AaptBuild constructs the apk out of the dex file and
the targets resource dir. Refactored the Aapt pipeline,
there is now an AaptTask base class. When calling
"bundle" on Android code you should now get an
unsigned .apk as output.

This also adds an AndroidIntegrationTest base class to
simplify Android integration testing.

I moved the 'package_path' method out of codegen and into
the AaptTask because all the aapt methods needed it. If
someone thinks it is truly useful to other codegen methods
I am open to moving it higher.

Testing Done:
CI.sh passes
Travis is failing elsewhere, so no Travis info.
Unsigned apks are indeed generated :)

Reviewed at https://rbcommons.com/s/twitter/r/859/