Enable Process Observer onProcessStarted broadcast

This change was previously reverted due to b/323925686 and has not been revisited since.

Looking at the bug, it seems like it is a concurrency issue where there are concurrent access to RemoteCallbackList's beginBroadcastInternal.

Adding synchronized block should resolve the issue.

Bug: 323959187
Bug: 323925686
Bug: 389579807
Test: atest FrameworksMockingServicesTests
Flag: android.app.enable_process_observer_broadcast_on_process_started
Change-Id: I706d8a3cda7418e30a95614a1c9303f0bec55537