Check if the uid still exists before attempting to remove app op state

This is a workaround to deal with a package manager bug around shared uids
and multiuser cases. In the short term it is safer to fix here but in
the next release the root cause is planned to be fixed so this will be
able to be removed. The UID_REMOVED broadcast sometimes is incorrectly
fired while the uid continues to exist so we need check if the uid still
exists before we continue to remove its app op state.

Flag: android.permission.flags.dont_remove_existing_uid_states
Test: Create app sharing system uid and install on all users and
      uninstall on one, observe that we stop processes the UID_REMOVED
      broadcast early.
Bug: 353474742
Change-Id: I5f2ffb47dada7513d808e22f2cd6f33faa65776f