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

public/property_contexts

@@ -110,7 +110,7 @@ ro.sf.lcd_density u:object_r:exported3_default_prop:s0 exact int
 ro.storage_manager.enabled u:object_r:exported3_default_prop:s0 exact bool
 ro.telephony.call_ring.multiple u:object_r:exported3_default_prop:s0 exact bool
 ro.telephony.default_cdma_sub u:object_r:exported3_default_prop:s0 exact int
-ro.telephony.default_network u:object_r:exported3_default_prop:s0 exact int
+ro.telephony.default_network u:object_r:exported3_default_prop:s0 exact string
 ro.url.legal u:object_r:exported3_default_prop:s0 exact string
 ro.url.legal.android_privacy u:object_r:exported3_default_prop:s0 exact string
 ro.vendor.build.security_patch u:object_r:vendor_security_patch_level_prop:s0 exact string