Camera / UidObserver: Add the ability to subscribe to specific UIDs

UidObserver sends updates about the state of all packages installed on
the system. In the case of the cameraserver, we only care about a
handful of them. The current status quo is to filter out these callbacks
but there is a significant IPC cost that is not addressed by that
approach.

This patch adds new entrypoints to ActivityManagerService to listen only
to specified UIDs. This set of uids can be updated dynamically.

Change-Id: Ic699526588a3f9157ca7d38b905f898876d86847
Bug: 274486653
Test: -- on physical device:
      -- testCamera2AccessCallbackInSplitMode x10
      -- ActivityManagerServiceTest
      -- ActivityManagerProcessStateTest
      -- ActivityManagerFgsBgStartTest
      -- UidObserverControllerTest
      -- Alternate focus in split screen between Camera2 + GCA x20