AudioService: SCO audio backward compatibility

After commit 25fc29b3, AudioManager.startBluetoothSco()
does not use virtual voice call mode anymore when starting the
SCO audio connection to the headset.
To help backward compatibility, this change makes that virtual voice call
is used if the request comes from an application targeting a SDK version
before JB MR2. For applications targeted to JB MR2 and above a raw SCO
audio connection is established.

Bug 8157702

Change-Id: If1ded2fd99b7ed76d2435d95ee03659e78a7882a