HFP: Only active BT device can hung up call

HFP: Only active BT device can hung up call

Some inactive devices can't send hung up key to host when
HFP connected and active call exist.
And other inactive devices can send hung up call by
sending hung up command.

Solution: To keep consistency of this behavior,
only allow active BT device to hung up call.

Test: Active device can hung up call and inactive can't hung up

Change-Id: Iec1ae689f1d40c15199707e9e925814f71ded619