MediaPlayerService: prevent audio_attributes_t race conditions

Access to audio attributes fields in Client and AudioOutput
 was not always locked.
Audio attributes field in AudioOutput cannot share the same pointer
 as Client because it can be indepently accessed. Save the
 attributes inside AudioOutput instead.

Bug 22672670

Change-Id: Ib1002b57b45cea44ff5e6eb115d581dc3beec006