Move all tethering functionality into TetherController.

This includes the entirety of NatController and relevant
parts of BandwidthController (specifically, getTetherStats
and its dependencies).

This will make the code easier to understand and allow netd to
privide a simpler API to the framework (e.g., by providing
higher-level methods that perform what today are multiple
operations from the framework's point of view).

It will also reduce duplication of state (e.g., interface pairs
kept by NatController partially overlap with downstream
interfaces kept by TetherController) and avoid dependencies
between controllers.

This CL makes no functional changes. The only code changes are
the ones necessary for compilation. Specifically:
- Move some methods into IptablesBaseTest.cpp so they can be
  used from two test classes.
- Change TetherController::iptablesRestoreFunction to the
  three-argument used by the erstwhile BandwidthController
  tests.

Where appropriate, variable names have been changed, but strings
that actually appear on device, such as iptables rule names, are
left as is and will be changed in a future CL.

Bug: 32163131
Bug: 64995262
Test: bullhead builds, boots
Test: netd_{unit,integration}_test pass
Change-Id: Idc80bfd424ce011826305f84b1cc98e741698601