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: I0ea1b53eb1272a8d83b63815fc0a05b73cef75f1