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
Merged-In: I3b91125b1a715690c2cd417b1e937e568c755d9f
Merged-In: I05de77d9b90d147bf1d6ee7f7ee19a049afddfa1
(cherry-pick of aosp I721aca4789ddfbee5a97316aae0b378d79ee2107)

Change-Id: Idfdd1b9cd5419c1f51f0fbb1eba2f36a9c12474b