DisplayManagerService: use per-pid freezer listener

Move to the new per-pid listeners for handling frozen/unfrozen
processes, which avoids sending binder callbacks to frozen processes
belonging to system UIDs.

DisplayManagerService has undergone a refactoring with respect to the
UID listener and the frozen process notifier.  The refactoring does
not change any behavior.  A flag controls whether the frozen process
notifier is actually used.

Flag: com.android.server.am.defer_display_events_when_frozen
Test: atest
 * DisplayEventTest
 * CtsDisplayTestCases
Bug: 326315985
Change-Id: I2f533d432d3f4cf5f2ee16d5e10f4609cbaa9c34