MediaRouter: save sessions in media router service

Each MediaRoute2Provider instance saves session infos
published by MediaRoute2ProviderService.

For that, MediaRouter2ProviderService notifies provider info and session
infos simultaneously and to minimize the number of binder calls,
schedulePublishState is added into MediaRoute2ProviderService.

The saved sessions can be obtained by calling MRM.getActiveSessions().

Test: atest mediaroutertest
 && manually call MRM.getActiveSessions()

Change-Id: I6938105f8d154ad9542f7e7c6210a451ee4ae90d