Add a density axis to tapas.
Add a density axis to tapas.

It allows overriding the density the app
is built for. Currently only used in the
GMS core APK's gradle file.

Change-Id: I3606df313a3110208cd8e6acade5f558261c921f
(cherry picked from commit 7e3d2341570681e566872216796dc4f79c8695ef)

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build