Separate route modification and update linkproperties

This is a no-op, preparatory refactoring to isolate route
modification functionality, enabling subsequent changes to move
routing control to ConnectivityService.

Test: atest TetheringTests:android.net.ip.IpServerTest
      atest NetdStaticLibTests:com.android.net.module.util.NetdUtilsTest
Bug: 349487600
Change-Id: I55af07968567b173dd9f97795600390439f1b927
3 files changed