Check MT call setup state before starting ring timer

Test: 1. Pair and connect to Bluetooth Headset.
2. Make an incoming call and accept it from phone or BT headset.

Failure:
User may hear ring although incoming call accepted.

Root cause:
There is a ring timer to send RING every 5 secs to connected
headset until the MT call is accepted. When the call is
accepted, we stop the ring timer and send call indicators.
In case, just at the same time the ring timer stopped and
call indicators sent, if ring timer expired, it will be started
again.

Fix:
Start the ring timer only if the call is not accepted yet.

Bug: 35415875
Change-Id: Ifbde9ac45ac406f46e418a8d20abff0fefe6d414