Telephony(MSIM): Default NW mode property for each slot

For dual SIM devices, add provision to configure different default
Network modes for each slot, using the existing system property to
configure this default network modes.

Bug: 28384694
Test: manual - Checked different default NW mode can be set
for each slot.
Change-Id: I72b11522cb51a425e28ddc407014387a20cb2cd7