[Media ML] Update MediaSessionManager#getActiveSessionsForUser

In order to make getActiveSessionsForUser, we need to remove
@UnsupportedAppUsage first.
Following UnsupportedAppUsage removal policy by adding
publicAlternatives to be gathered by internal tool and published
on the Android Developers site.

Bug: 160666400
Test: build
Change-Id: I13d639489091087a5c00e81071f36740002ffd9a