Fix deadlock in CameraAccessController

Use separate lock for mObserverCount and to ensure the listener
is only registered once.

This breaks the resource allocation cycle and prevents deadlock
described in b/265618228.

Bug: 265618228
Test: atest VirtualDeviceManagerServiceTest CameraAccessControllerTest
Change-Id: Ifb8345319b367aa6492a46bea404658041272cc0