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
efe4374eb4