Fix issue that ro.telephony.default_network can't be read per slot

"ro.telephony.default_network" can define as comma-separated Sting per
slot for multi SIM device. However, it cannot be read correctly due to
it defined as Int in property_contexts file.

Bug: 110626665
Test: manual - Checked the ro.telephony.default_network can be read per
slot for multi SIM device.
Change-Id: I900620e46c819c14bf339751f00a1db1473fd45f