[Media ML] Replace user-related hidden APIs

MediaSessionService keeps track of users being added and removed from
the system. It also considers "managed" users (work profiles) to be
part of the "manager" user, since they will always be running
simultaneously and therefore should share a common logic for session
management (e.g. dispatching a media key event to the most recently
active session).

Thus, MediaSessionService needs to know what users
are "managed" and who their "manager" user is. This CL replaces the
existing hidden API usage with existing public/system APIs.

Bug: 175216227
Test: Manually test adding managed/non-managed users
Change-Id: I0ce807e5bdcccee7ae99e29c0a0a52b6dadc8b57