Merge "Find a background to animate when launching a dialog or activity" into...

Merge "Find a background to animate when launching a dialog or activity" into tm-dev am: 18906f73 am: b32b4d58

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/17075008

Change-Id: If213140deb9f844ae0f15725ca18cd6d33e57ee6