AudioFlinger: fix tracks ready for mixing logic.

Commit fec279f5 broke the logic allowing to wait for an application
to provide frames for mixing in the case of several active tracks.

This was causing audio gaps when playing music and superposing a
sound Fx (keyboard clicks...).

Issue 6185007.

Change-Id: Id0fad150d0b615646d6b1387c0de8ca944d228f6