Migrate MediaController to use OnBackInvokedDispatcher

The OnBackInvokedCallback will be called when an app is requesting the new back dispatch behavior. Otherwise we keep dispatching KEYCODE_BACK to MediaController.

Test: In BackTestApp, try dismissing media controller by swiping back, both in new and legacy back dispatch.

Change-Id: I9ad3c199277838639c875c78a3a8dfb0963b3ca4