Use beta package in Gradle build for beta flavor

Also added alpha flavor to Gradle build, using alpha package.
Currently this still assumes that the prepare-release.py
script is used before the build is run.

Once we abandon Maven we can substitute most of the
prepare-release.py functionality with Gradle configs.

Change-Id: I7d0f748c6b913fcc6ce69e1ac221ef344634b9b6
(cherry picked from commit 2c1ce18de7f4d99dce3616963d4a564383679112)
Merge "Use beta package in Gradle build for beta flavor"