HFPClient unhold conference call

Allow a conference call with more than one party to be unheld.  Clean up
hfp_enable logic to prevent it from being set twice in a row.

Bug: 63412812
Test: Create a multi party call on a GSM network and hold the phone
call, upon resuming audio is routed properly.

Change-Id: Ib31259eba0bbb171015f26b30a415e0539b3d0ec