Fix multichannel downmix pause bug on video player

Pausing a video player will cause the track audio mixer to be
 disabled, which causes the downmixer to be deleted. When reenabled,
 the track channel mask hasn't changed but the downmixer is there
 anymore.
Fixed by:
- instanciating a downmixer when the AudioMixer track
 gets initialized (in getTrackName(), now taking a channel mask
 as input), and deleted when in deleteTrackName().
- when the channel changes on a track, check whether it
 needs a downmixer or not. Preparing a track for downmix
 automatically removes the old downmixer if there was one.

Also: initialize the track downmixerBufferProvider field
 when AudioMixer is instanciated, so we can safely call
 delete on it in AudioMixer's destructor, in case
 deleteTrackName() wasn't called before the mixer was
 destroyed.

Change-Id: I589b0781cda5b3c82f85b561c52b08546cac21f8
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index c674c24..99dcf45 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -2664,13 +2664,7 @@
 // getTrackName_l() must be called with ThreadBase::mLock held
 int AudioFlinger::MixerThread::getTrackName_l(audio_channel_mask_t channelMask)
 {
-    int name = mAudioMixer->getTrackName();
-    if (name >= 0) {
-        mAudioMixer->setParameter(name,
-                AudioMixer::TRACK,
-                AudioMixer::CHANNEL_MASK, (void *)channelMask);
-    }
-    return name;
+    return mAudioMixer->getTrackName(channelMask);
 }
 
 // deleteTrackName_l() must be called with ThreadBase::mLock held