MediaRouter dialog

Add the dialog behavior for MediaRouteActionProvider/MediaRouteButton.

Still TODO:

* Switch audio icon based on source; speaker/bt/user

* Rig up volume slider

* Rig up item icons

* Rig up group button for groupable categories

* Make grouping work

Change-Id: I3f992516b184d5ae940ddb7bbb7f94ff58914589