Unbreak TetherInterfaceStateMachineTest.

This was broken by the implementation of IPv6 tethering, which
relies on various private classes which end up calling unmockable
final classes like NetworkInterface.

Making everything public like this is almost certainly not the
best way of fixing this test, but on the other hand there is
value to the test continuing to pass.

Test: runtest frameworks-net  # Everything passes.
Test: IPv6 tethering continues to work.

(cherry picked from commit 7e0eeca8)

Change-Id: I8a3cf466871c026f4ae0f5cfa73071338cdf5b7e