BouncerKeyguardMessage doesn't always get cleared when being set to null. am: 08241b1e6a am: afe3261ed6

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/20244730

Change-Id: If3ff4f5a6be87defad807180cfe14341e7e27965
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>