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: I28996d425838728955f01eb1a00acf6e6dc2dea1
Signed-off-by: Gaurav Kumar <gaurav.kumar@broadcom.com>
Signed-off-by: Pierre Couillaud <pierre@broadcom.com>