Fix bug with discarded AudioRecord::read count

Formerly, if an AudioRecord::read() got a timeout on obtainBuffer()
after already successfully transferring some data, then it returned zero.
This had the effect of discarding a partial transfer, which resulted in
a gap in the audio data delivered to the app.  Now if a timeout occurs
after a partial transfer, it returns that partial transfer count so that
no data is lost.

Change-Id: I0d9c2f4e495a400b56ef916a06613ba26537ca97