Fix deadlock b/w ActivityManagerService and MediaSessionService

While handling adjustVolume and finishReceiver at the same time,
system_server gets into deadlock situation where
ActivityManagerService lock and MediaSessionService lock are
getting acquired in reverse order. To fix this,
adjustSuggestedStreamVolume() is shifted to MediaSessionService's
Handler thread.

https://code.google.com/p/android/issues/detail?id=213014

Change-Id: I3125c890546c8ab7c4df530bb68ba87cd688f8ce