Merge "Make sure we start new activity only in UI thread." into tm-qpr-dev am: fd2471b9

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/21484427



Change-Id: I851ac58eb0dafa863bbf6efb6e19299d04ae3e2e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>