Add Listeners to PlatformCompat

This allows doing more work except killing the app upon change override.
Up to one listener per change id is allowed. Currently intended for
SeLinux usecase only (for data relabeling, see http://ag/9635653).

Test: atest FrameworksServicesTests
Bug: 143539591

Change-Id: Ib2110a31d4f521fc8054620a9edf902fa5bba501