Allow root uid to listen for uid state changes across users.

ActivityManager.checkUidPermission() doesn't exempt root uids
similar to other permission checking methods. While we
investigate why, switch to a different permission checking
method.

Bug: 385438310
Test: atest ./tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Flag: EXEMPT bugfix
Change-Id: I6f254f08f3dafe0156f2f6a77f3ef7ad461a0e65