Add tests for IPv4 of dual stack tethering.

Also refactoring some Tethering and TetherInterfaceStateMachine calls
to address testability issues.

This is in preparation of other work to have IPv6-only or 464xlat
tethering working.

Test: runtest frameworks-net
Bug: 38218697
Bug: 64382985
Bug: 64976379
Bug: 64995262

Change-Id: I721aca4789ddfbee5a97316aae0b378d79ee2107