hal: Fix for no audio on BT headset till waiting call is ended.

-Issue: Voice call audio is not routing to BT headset
 though connected, till the waiting call is ended.
-Rootcause: During voice call, if another call is received, the
 audio mode is changed to MODE_RINGTONE. Due to this, the primary
 output on which the voice call is setup, is not able to find
 new device as BT in getNewdevice().
-Fix: Avoid mode change to MODE_RINGTONE when voice call is already
 active.

Change-Id: I38582e2500ff4c44eb5ac45dab3aab35606d1c7d
CRs-Fixed: 662883