Bug 875677 - Modify the logic of sending 'callheld' indicator to fit HFP 1.6, r=gyeh

According to HFP spec 1.6, we should update callheld status very carefully when
the status of other calls are changed.
Bug 878005 - Send system message 'ATD>' to Gaia instead of sending 'BLDN' when receive 'ATD>' from headset, r=gyeh