AudioFlinger: Fix re-use of fast-track

When a fast-track using shared buffer is restarted, we need to call
obtainbuffer if frameReady has already become zero, even if track is
still active. This is required to reset mFramesReadySafe. Otherwise
mFramesReadySafe remain at 0 and fastmixer can't consume data from
the track when it is re-used.

Change-Id: I5d6f364f8f31baad3341bd4f51bf8a8b147cd7d7