BT-A2dpSink: Prevent force close during pairing

Use case:
1) Open BT test app and select A2dp Sink radio button.
2) Receive paring request from Remote device which
  support avrcp source.

Observed result:
Setting App crashed.

Root cause:
Seems race condition issue, sometimes Bluetooth profile
manager not able to return valid A2Dp instance.

Fix:
Add extra check with A2dp profile instance to
prevent force close in settings app.

CRs-Fixed: 2251058
Change-Id: Ib21876a783de905ca65f7d1ca439fac3139d60d6