Respond ATD request with OK if dialing success.

Use case:
Dial a call from car kit when there is an active call

Failure:
OK is not received for ATD

Root casue:
In case when remote sends CHLD command to put an active call on
hold before dialing a new number, if call held update for 1st call
comes before dialing update for 2nd call, then AG doesn't send any
response to ATD. This may cause some carkits to behave bad.

Fix:
Do not reset mDialingOut flag if call held update for 1st call
comes before dialing update for 2nd call.

Change-Id: I7a09d3137a8a956d52605694a63d080f439c0d59