Revert input buffer count for the SoftAAC* decoders back to 4.

This improves the netflix bandwidth change responsiveness.
With the old count of 32 the audio decoder running at a higher
priority would starve the video decoder for about 1 sec while
draining the input data.

I haven't been able to break any stream playback using MediaCodec
using the lower count, even though I'm sure I had a good reason
to increase it in the first place (see bug 6478823)

Change-Id: I7fd3e71794b0cc1570ea9d27d1d869ab1e1750c4
related-to-bug: 6685031