DO NOT MERGE

System bar now renders the back button for recents

We used to render the back button in Recents
itself, but that had an inconsistent visual
experience.

As a part of this change, add the early beginning
of support for private status bar APIs for dimming
navigation buttons.

Bug: 3448536
Change-Id: Ie64369e3df3178178f35b56763ea96d380b6ddd4