Remove potential cause of underrun.

1. This part of ProcessAudioBuffer function can make underrun issue.
   Because AudioTrackThread::threadLoop's default case is 'pauseInternal'.

2. Without removed part : If AudioTrack obtain small buffer, then it can request small decoded data to player service without waiting.

3. We already have 'waiting codes' in 'AudioTrackshared.cpp ClientProxy::obtainBuffer', it using '__futex_syscall'.
   If we got 'full shared buffer', then 'ClientProxy::obtainBuffer' waits 'ServerProxy::releaseBuffer'.
   It means, there are duplicated 'waiting codes' for ProcessAudioBuffer.

So I removed this part for preventing underrun issue.

Change-Id: Idcca686054d92a4d123873b78b3b9c2e87d3bdca
Signed-off-by: Jinhyuk Seong <jin.seong@lge.com>