BG-FGS-Launch uses the same grace period mechanism as BG-Activity-Launch.

WindowProcessController.areBackgroundActivityStartsAllowed() has the 10 seconds grace period,
starts from last activity's launch time or finish time.

Expose this method as an ATMS interface for AMS to call, to determine if
BG-FGS-Launch is allowed.

Bug: 171305836
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Change-Id: I30706dc1950bf5d49926a8e26c0493d9e168eb1b