Merge "Run addUidToObserver and removeUidFromObserver on the handler thread." into udc-dev am: 0bc1b58227

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/23591556

Change-Id: Ief96b5fc1ef6bd40f17f1f2d316b606294b640ee
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>