Merge "Polish activity transition for letterbox" into tm-qpr-dev am: c3a33cde am: 19b1ed40

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/18320843



Change-Id: I09ee2e78a50596473b7b4488ec6e99d3b453a990
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>