Fix to fetch conversation sub correctly and prevent NPE

If the sub is not in LCH and it has a active call then it should
be considered as the conversation sub.
Add null check to avoid NPE due to phone account being null
when there is no sim card.

Change-Id: I5f1b5a978bb2b0f84dfbedb9e4984ba734963496
CRs-Fixed: 764545,764581