debug utils: use ffmpeg for WavWriter

The previous writer was not working, the format was not
recognized by most players.
The new version uses ffmpeg APIs to produce working wav files
(tested stereo s16 and mono f32).
The API remains the same and it should be a drop-in
replacement for existing code.

Change-Id: I328e32427d94a44085ddb1567b5a1bbee5a6a8bc