Make sure all data network carrier configs have default value

This cl sets the default value (all empty string) for the following
carrier configs which didn't set default value:
- KEY_CARRIER_DATA_SERVICE_WWAN_CLASS_OVERRIDE_STRING
- KEY_CARRIER_DATA_SERVICE_WLAN_CLASS_OVERRIDE_STRING
- KEY_CONFIG_PLANS_PACKAGE_OVERRIDE_STRING
- KEY_CARRIER_NETWORK_SERVICE_WLAN_CLASS_OVERRIDE_STRING
- KEY_CARRIER_NETWORK_SERVICE_WWAN_CLASS_OVERRIDE_STRING,
- KEY_CARRIER_QUALIFIED_NETWORKS_SERVICE_CLASS_OVERRIDE_STRING

Without default value, the configs will not be returned from APIs
like CarrierConigManager.getConfigForSub if default and carrier
config app doesn't override them. It may cause caller
process crash if not handled properly.

Bug: 261776046
Test: atest FrameworksTelephonyTests
Test: Basic phone functional test (activation/call/data/messages...)
Change-Id: I633799e266df2a692bf6b90f6d1e0499477bdaf3