Inline tetherInterface and untetherInterface

This is a no-op refactoring which inline the functionalities
to IpServer, enabling subsequent changes to move
networkAddInterface part to ConnectivityService when the
tethering is enabled.

Test: atest TetheringTests:android.net.ip.IpServerTest
      atest NetdStaticLibTests:com.android.net.module.util.NetdUtilsTest
Bug: 349487600

Change-Id: I6d0e5d349691df7a15bee852d5fd1dd19f5da6f6
3 files changed