libmediaplayerservice: Use default buffer size for Audiocache with Awesomplayer

-Current buffersize is equal to the heap buffer shared to copy
decoded pcm data.
-This causes the postEOSDelayUs to be equal to the clip duration,
for AudioCache output.
-This results in AudioCache Callbackthread to busy wait that long.
-Use default buffersize of 1024 frames, for AudioCache when used
with Awesomplayer, this ensures that postEOSDelayUs is max equal to
1024 frame duration.

Change-Id: I8c1debea2d63942b66219c364efb317c61bf0169