libstagefright: Fix crash in media server mpeg2 playback.

 -There is a check in dequeueAccessUnitMPEGAudio function in
  ESQueue that time stamp returned by fetchTimeStamp is always
  greater than 0
 -In the current scenario, valid time stamp set on the first
  frame is getting replaced in the sub sequent code by -1.
 -Fix is to reset the flag after first frame is identified

Change-Id: Ifcbb8ed95751efb8aded6cdeb217fab82762a782
CRs-Fixed: 512218