Ensure that we register the media session listener for the current user.

- Use the per-user calls when fetching/responding to media session events
  otherwise it will fallback to using the process user.

Bug: 64315017
Test: Launch secondary user, enter PiP with media session and no custom
      actions.  Ensure the media session buttons show.

Change-Id: I52152223e91f0256ac4135616694e2a293947e3e