Merge "Make UserTrackerImpl use UserSwitchObserver instead of broadcast." into tm-qpr-dev am: 8fd6cb70f0

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/21024523

Change-Id: I3af32d2ecff028eb35cd4a39840ee162bcf06bae
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>