Fix IpConfigStore dependencies on framework-connectivity

This change fixes IpConfigStore dependencies on framework-connectivity
hidden APIs.
- Use the version of SystemApi to new RouteInfo.
- Inline the implementation of isIPv4Default() directly because
  isDefaultRoute() is a public API so it can be used in this class.

(cherry-picked from ag/13945685)
Bug: 178777253
Test: FrameworksServicesTests:IpConfigStoreTest
Merged-In: I5ed1bc8bc8bee5b0c795fd8577a2d64628998e51
Change-Id: I5ed1bc8bc8bee5b0c795fd8577a2d64628998e51