Add unit test for ActivityStackSupervisor#testApplySleepTokensLocked.

This change adds a unit test to make sure the supervisor interacts
properly with the stacks when this method is invoked under various
conditions.

Change-Id: If0fbacfa4c8bc18ae85183a22edeec54d6f90dc4
Fixes: 74597379
Test: atest FrameworksServicesTests:com.android.server.am.ActivityStackSupervisorTests#testApplySleepTokensLocked