Delay setting of active subscription

In case of MT call, delay setting of active subscription till we send
onIncoming to InCallUI. Until onIncoming comes, there is no MT call
in IncallUI call map, hence active sub shouldn't be set much earlier than
sending onIncoming() in order to avoid timing issues.

Change-Id: I7a6ff5bed017d51e540f207093565310e6e9e511
CRs-Fixed: 655859