Fix a couple issues with AppOpsUidStateTrackerImpl

1. When a process goes to NONEXSISTENT we should dispatch the uid state
   change if necessary
2. Dispatch the new UID state on changes, not the last one (how did this
   not break anything?!)
3. Fix issue in logic for detecting change between fg/bg uid states.
   This probably didn't break anything since all appops that currently
   care about foregroundness today also have capability that also
   changes.

Test: CtsAppOpsTestCases, CtsAppOps2TestCases, AppOpUidStateTrackerTest
Bug: 241447638
Change-Id: Ib1b93148669ee04d7c16195540906b388550ee68