Bluetooth-OPP: Clean up OPP threads during user turned OFF BT

Cleanup OPP threads during Bluetooth is turing OFF. Otherwise,
if threads are in sleep during turn off and try to resend the failed
transfer immediately after Bluetooth is turned ON transfer won't
start and there are no status update in UI.

CRs-Fixed: 766506
Change-Id: Ibf06bf25af587df0982845d3300fced3c56bbd46