Move logic for handling uid observers outside AMS - part2

- Do not depend on AMS lock anymore for handling uid observers.
- Update AMS to not directly access internal state of UidObserverController.

Bug: 163963556

Test: atest tests/app/src/android/app/cts/ActivityManagerProcessStateTest.java
Test: atest services/tests/servicestests/src/com/android/server/am/ActivityManagerServiceTest.java
Test: atest services/tests/servicestests/src/com/android/server/am/UidObserverControllerTest.java
Test: atest tests/ActivityManagerPerfTests/tests/src/com/android/frameworks/perftests/am/tests/OomAdjPerfTest.java

Change-Id: I7ba7552eace7e713f882af36bd507f108f398d7a