Add CompatChanges API.

This wraps our existing APIs for isChangeEnabled, and is exposed for
mainline modules to use.
Also upgrading PlatformCompatChangeRule to androidx to match with
frameworks coretests.

Test: http://aosp/1203413
Test: atest android.app.compat.CompatChangesTest
Test: atest com.android.tests.gating.PlatformCompatGatingTest
Bug: 142650523
Change-Id: I12b9436c2ea955e9b74228f150b872d6eb3a1081