audioflinger: decline track to be offloaded if there's active one

-If multiple offload tracks are activated at the same time,
 status of the tracks can be affected by each other and results
 in unexpected behavior
-Deny subsequent track to be offloaded and fall back to sw decoder
 if there's already actived one

Change-Id: Idaa9213d54b9bcdd0e5c25c1dec516d994ae006f
CRs-Fixed: 704382