libaudioflinger: add stock resampler in pcm offload path

- when samplerate changes in b\w playback, resampler should be plugged-in
  so that the o\p sample rate remains the same to h\w
- this resampler is only enabled on 16 bit PCM path, when channel count is
  <= 2 and channel count doesn't change mid-stream
- once the sample rate resets to original\initial sample rate, resampling
  should not be done
- by default this feature is disabled

Change-Id: Ic27bbf9174d0a83297ebc4418eebdc8850272a6b