User OverlayIdentifier instead of @ColorInt

Migrated the feature flag to the new FeatureFlags infrastructure and
replaced the @ColorInt interface by a more flexible OverlayIdentifier.

Test: atest ThemeOverlayApplierTest
Test: atest ThemeOverlayControllerTest
Bug: 171916625
Change-Id: I701fd5bda2aca67c069f25a22d397d56b077d450