frameworks/av: Add output close in AudioTrack destructor for offload case

Close audio output handle inside AudioTrack destructor for
offload case to make output handle close/open independent of
each other and support multiple concurrent offload plyabck
session.
This is controlled runtime based on system property:
audio.offload.multiple.enabled

Change-Id: I98b3a6fe03bfddca2ed01a2adc5a33e4ad540e67