AudioMixer: delete reformatBuffer provider in proper order

 If mixer is creating reformatBufferProvider and downMixerBufferProvider
 while adding a track it first  creates  downMixerProvider
 then reformatBufferProvider. While deleting track it first
 deletes downMixerBufferProvider and then reformatBufferProvider,
 which should be in reverse order, the object created last should
 be deleted first.

Change-Id: I844e7862280fe37c3167b31e92bbb27aa9463315