Ensure taskbar background always transparent in notification tray. am: 84261f9d

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/21593273



Change-Id: I301369bdce2bf00ef00d95d75ae91365022e4d6a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>