CarrierConfig: fix giffgaff voicemail number

Although CarrierConfig/assets/carrier_config_23410.xml has
set default_vm_number_string=443 for gid1=508, it doesn't work
since the actual SIM has gid1=508ffffff... So we have to workaround
by using spn=giffgaff in overlay vendor.xml.

Bug: 64322610
Test: manual
Change-Id: Ic99734526fb1663700e34779a3740b9efb2e5ac4
1 file changed