Merge "Dim background when group notification expanded" into sc-dev am: 9c4b4c34 am: 748e2433

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/15022247

Change-Id: Ie395c612c55e57d7c891a2685d0b0f818c2bfb84