[bluez] Work around non-atomic ofono ReleaseAndAnswer. Fixes JB#11816.

Ofono ReleaseAndAnswer method is implemented so that the second call
state becomes incoming before first call becomes disconnected; this
causes bluetoothd to emit RING signals while the first call is still
considered ongoing.

Workaround the problem by calling Hangup and Answer separately instead
of using ReleaseAndAnswer.