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

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

Change-Id: I3b35c1c3fb66f891ef3f34aebcf43ee00fd7d279
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>