nuplayer: ignore openAudioSink while waiting for teardown completion

- AudioSink is reopened on AudioTearDown event as well as on
  decoder output format change. These events are from different
  contexts, independent of each other and can occur concurrently,
  doing set of thread-unsafe operations on audioSink, which
  sometimes may result in fatal error.

- Any request to openSink, after teardown event is received but
  before its completed, is redundant since once teardown is
  completed sink is anyway going to be opened again. Also
  ignoring sink reopen while in teardown would avoid concurrent
  processing of thread-unsafe operations on audioSink.

- Update nuplayer to prevent reopening of sink while teardown is
  still going on.

Conflicts:
	media/libmediaplayerservice/nuplayer/NuPlayer.cpp

 CRs-Fixed: 978013

Change-Id: Ieb3825f133c2099e07d6e92ef101dd5156af8a20