libstagefright: Fix for FPS drop issue during A-V playback.

Issues:
-The AAC decoder was not updating the timestamp when EOS is reached.
-Logic to smoothen the real time update in AudioPlayer uses system
 time. This introduces corrupt timestamp during EOS.

Fix:
-Update the timestamp in AAC decoder when EOS is reached.
-Extrapolate realtime using system time in AudioPlayer when EOS is
 reached. Cap the value to realtime if  extrapolated time becomes
 greater than realtime.

CRs-Fixed: 384183 515066
Change-Id: Ice54501436431d2527fcd3d710d65d9732fcffdd