MediaRouterService binds services when necessary

MediaRouterService maintained bindings to provider services
once those are established.

With this CL, it only binds services when there is a non-empty
discovery preference set by a foreground app or an app is casting.

MediaRouter2Manager#startScan and #stopScan are added
so that system UI can force the service bind to the services and find
remote devices to cast.

Bug: 169575701
Bug: 172920557

Test: manually and CTS
Change-Id: I4a47fdb1c9fe05a04d26950485833c9cbfb91a69
(cherry picked from commit 9f889ca4)
(cherry picked from commit 602b168f)