Merge "Run addUidToObserver and removeUidFromObserver on the handler thread." into udc-dev am: 0bc1b582 am: 4ceab125 am: c892dc23 am: 356f68e0

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



Change-Id: I0def7e91457848e9d27d3ab016527f4d5eccf9c5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>