Minor refactor of ChangeReporter and PlatformCompat

Move all StatsLog generated constants into ChangeReporter, to make
re-submitting http://ag/10194708 easier.

Bug: 145952197
Test: atest PlatformCompatTest ChangeReporterTest
Change-Id: Iccff9582099c58f070a6b6aeb1d2588b0fafa90a
4 files changed