Fix issue 2306779: Runtime restart - Init failed at android.media.ToneGenerator.

The ToneGenerator failed to initialize because no more tracks were available in AudioFlinger mixer.

All tracks were used because the duplicating output was failing to free the tracks on audio hardware output mixer when exiting due to a misplaced test on output activity: output tracks where only freed if the duplicating output was active when exiting.

The fix consists in freeing the output tracks when the duplicating thread is destroyed without condition.