Fix more locking issues identified by @GuardedBy.

- Add one missing locking for mRegistry.removePermission().
- Add @GuardedBy annotations for *Locked methods.
- Remove unused mBackgroundPermissions.
- Don't lock access to mOnPermissionChangeListeners because
  RemoteCallbackList has its own locking.
- Lock access to mPrivappPermissionsViolations.

Bug: 158736025
Test: presubmit
Test: errorprone build logs/warnings.html
Change-Id: I356b0d3100fa555b163e6c40dad78f4b516c521b