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.

Bug:38373601
Change-Id: I1c7e1d3b766dd5a84a790f7742c11f840fe22889