[bluez] Explicitly hold current call before dialing a new one. Fixes JB#...

...11613.

Calling ofono voicecallmanager Dial method should result in active call
being put on hold and the new call dialed, but due to some issues the
call state bounces between active and held once too many times. This
causes problems with headsets monitoring the hold status.

Fixed by explicitly placing the active call on hold and dialing only
when it reaches held state.