BrowseFragment: headers transition back press support.

Added a default support for "headers transition on back pressed".
Also provided BrowseTransitionListener and startHeadersTransition() for
the rare case if app has its own onBackPressed() handling (e.g. PlayStore).

b/13628758

Change-Id: I8c70171d0353d6b212fec9e9b0e4739b9f0d2138
3 files changed