Merge "Fix NPE from BackAnimationController#setBackToLauncherCallback." into tm-dev am: a671783048

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/18785512

Change-Id: I733cd5b229c23ad2c8bf166e938eb376412449fe
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>