Run addUidToObserver and removeUidFromObserver on the handler thread.

If we're in dispatchUidsChanged while another thread is calling
addUidToObserver or removeUidFromObserver, there will be two active
broadcasts for mUidObservers. Move this processing to the handler thread
to eliminate the possibility of this race condition.

Bug: 285982408
Test: Ran testCamera2AccessCallbackInSplitMode
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:345a42452b5c10f05fb5fb2d1753712ac030a731)
Merged-In: I9d64978415ebb42f194936ec56ba9933ed1533e7
Change-Id: I9d64978415ebb42f194936ec56ba9933ed1533e7