MediaRouter: Introduce discovery request

Providers are notified of change of discovery request
to respond properly.

Whenever a media app registers a callback it should provide
discovery request which includes route types and a way to scan.

MediaRouter combines all discovery request into a single request
and sends it to the media router service, which combines requests
from all clients and notifies provider of the combined request.

This CL also changes control category as route type.

Bug: 139033746
Test: atest mediaroutertest
Change-Id: I5ca37683ae0761ff587c1c920e32fd6d768a2cd3