Telephony: On demand DDS switch uses preferred sub

Pass user preferred subscription id on oem hook request
for on demand DDS switch. RIL uses this information
to send DUAL_STANDBY_PREF_REQUEST down to the modem, based on
which the modem stack is selected.

CRs-Fixed: 806933

Change-Id: I883bce818652253feae14a3be52eb4d510731277