Fix the call is soundless due another call is active

BT may unhold the held call while a dialing call is ongoing,
so that both calls can be active shortly leading to fail
the sound output.

To fix it, block the processing of unholding while an outgoing
call is present.

Test: manual
Bug: 147027350

Change-Id: If31744f0eceb5bf6957133251abcf191c4d12a4d