Fix up 'launch player on headset connection' option.

- don't crash system_server if no music player can be found
- use the active user's setting instead of the owner's setting

Change-Id: I38a07fe5c8c3d67ececd0768e1fcc01a6cf7fa5a
Merge "Fix up 'launch player on headset connection' option." into cm-10.2