Make MediaRouter UI more robust around route count changes

Improve the API around ActionProvider visibility overriding. Allow the
application to notify whatever is hosting the ActionProvider that
visibility has changed in a way that is friendly to alternate support
library-style reimplementations of MenuItem.

Allow MediaRouter.Callback implementations to add or remove themselves
or other Callbacks during dispatch of callback events.

Make MediaRouteActionProvider track the visibility of corresponding
menu items more accurately.

Change-Id: Ic7ddb6a87c3637904750d2661e4a9fa323b09ea0