Merge "[User Switcher] Add user and guest on bg thread." into tm-qpr-dev am: 57f5c6bcf1 am: 76b091fd09

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/19324250

Change-Id: I87b626f671cf38d753d057d4d7e4da345e6c2fd6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>