Merge "Dim background when group notification expanded" into sc-dev am: 9c4b4c34 am: 748e2433 am: 9b7bb090

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

Change-Id: I679eef2a9e8b9f795313a0df78d51f2105793cf8