Fix A2dp crash when re-starting Bluetooth services

When the Bluetooth process has died, the services will be restarted to
recover from a crash. This could cause an exception in the A2dp service
because the service maybe stopped without having successfully been
started before. Added null-check to prevent an exception during the
restart.

Bug: 16529242
Change-Id: I57c99ec72c0ba09c6c5325b262e83618aacb3e39