AudioMixer: Remove tracks from enabledTracks after reseting outTemp

If any track goes through AudioMixer::process__genericNoResampling, and
its getnextbuffer returns NULL, Then that track is removed by AudioMixer
from enabledTracks.

Thus if all tracks getnextbuffer return NULL, Then this function doesn't
reset outTemp and last buffer in AudioFlinger's mMixBuffer will be
repeated and noise is observed.

Remove tracks from enabledTracks after reseting outTemp to zero, so that
process__genericNoResampling will reset outTemp and noise won't appear.

Bug: 12450065
Change-Id: I1ccac7ee4a3bf7fd930254356c072099e11e7c19
Signed-off-by: Gaurav Kumar <gaurav.kumar@broadcom.com>
Signed-off-by: Pierre Couillaud <pierre@broadcom.com>