RecordBufferConverter: Release converter buffer after conversion

Required to ensure the RecordThread understands the true
number of frames consumed / available.  Otherwise the RecordThread
thinks there are more frames than actually present, causing
a buffer provider underrun in the AudioResampler and the
resampler to reset.

Test: BT phone call, Tee sink
Bug: 130419319
Change-Id: I96fac6f52233e8519c0cc098329e5b9b89c1dd89