Add app compat flag to enable FGS BG-launch restriction.

Add a app compat flag FGS_BG_START_RESTRICTION_CHANGE_ID to
enable FGS BG start restriction for any app. Default is disabled. To turn it on for
an app, go to Settings --> Developer options --> App Compatibility
Changes, find the app and turn on this change.

Bug: 170668199
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java#testFgsStartFromBG2
Change-Id: Ia54de45f9a417e287dcba19b9f09e0f63da953f1