Move logic for handling uid observers outside AMS.

Bug: 163963556

Test: atest src/android/app/cts/ActivityManagerProcessStateTest.java
Test: atest services/tests/servicestests/src/com/android/server/am/ActivityManagerServiceTest.java
Test: atest src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Test: check activity dump contains uid observers info

Change-Id: I1292decd1f24b6cbae094a909662aa795dd054a4