Fix bug in WAVExtractor for 24-bit per sample wav

support stereo/multichannel 24-bit format
kMaxFrameSize is fixed to 32768. When converting 24-bit to 16-bit,
number of samlpes is maxBytesToRead /3.
In this case, if the maxBytesToRead is not multiple of 3,
pcm data is messed when converting.

Bug:7630939
Change-Id: If3d339a86fa6025d489ab44e771378bc0c39083e
Merge "Fix bug in WAVExtractor for 24-bit per sample wav" into jb4.2