Add a unique build number to every build

SDK-readable via android.os.Build.INCREMENTAL

Change-Id: If9885adebe12a56b2b2e353285721be695bc411b

Conflicts:
	envsetup.sh
Fix exporting the unique build identifier

Change-Id: Ie58e981b7e83a4351d137dfd8995dce7d365d950

Conflicts:
	envsetup.sh