A2DP: Fix NPE issue while BT turning off

Steps:
1. Connect DUT to MecApp.
2. Close Mecapp.
3. Disconnect the Mecapp in the BT paired list on DUT.
4. Then quickly turn Bluetooth off during disconnecting
   Mecapp from DUT.

Failure:
BT crash while quickly turning off BT. And error
message "Bluetooth Share has stopped" pop-up.

Root Cause:
In Some conditions A2dpService is NULL while
BT turning off.

Fix:
Before get A2dp connected devices, check A2dpService
value to avoid NPE.

Change-Id: I65d658da81fdb48ba40315d8cb176b9f3427bede
CRs-Fixed: 1088247