Merge "Letterbox: Display rounded corners behind navigation bar instead of...

Merge "Letterbox: Display rounded corners behind navigation bar instead of above." into sc-v2-dev am: e6fb34bd am: c9e8de5b

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/15934232

Change-Id: I085c8371bccee39575c85f8f78fe79dc44027551