Media: Add ClientRecord in MediaRouter2Service

ClientRecord is a record for media router client.
Save the client info on MediaRouter2Service to make setControlCategories
work and MediaRouter2Manager get the callback
onControlCategoriesChanged.

Test: atest mediaroutertest (after atest mediarouteprovider)

Change-Id: Iaf79242e18a699e339e91c8b0baceb767dc36bd4