In S, the behaviour of createTunInterface and createTapInterface
changed so that they bring up the interface before returning it.
This makes it difficult to test code that brings interfaces up
itself, such as IpClient or EthernetManager, because the tests
cannot predict whether that code will see the interface up
or not. This leads to flaky tests and can even make it impossible
to reliably test some behaviour.
Add a method that allows the caller to specify whether to bring
up the interface or not.
Test: new codepath tested by other CL in topic
Test: existing codepaths already well-covered
Change-Id: I0f7698f4dad132f201db4203e65a78c6af564ab2
333d5e52f5