Get default from CdmaSubscriptionSourceManager

Initialize mCdmaSubscriptionSource with value read from
CdmaSubscriptionSourceManager instead of assigning with
SUBSCRIPTION_SOURCE_UNKNOWN.
This is needed, since in a scenario where UE is powered in APM
and VoWifi PDP activation bieng attempted, getOperatorNumeric()
from phone object fails, since current
logic in GsmCdmaPhone allows read of operator numeric only when
mCdmaSubscriptionSource is not SUBSCRIPTION_SOURCE_UNKNOWN which
subsequently results in PDP activation failure.

Change-Id: I1c7e1d3b766dd5a84a790f7742c11f840fe22889
CRs-Fixed: 1043095