Add test coverage for explicitlySelected networks.

Also, make all tests start with mobile data always on disabled.

This is because some tests only pass when mobile data always on
is disabled. This doesn't cause any problems when running all
the tests in the file, because these tests are always run after
one or more calls to tearDown, which disables mobile data always
on. However, it does cause issues when those tests are run alone.

Test: new test passes 50 times in a row
Test: ConnectivityServiceTest passes
Change-Id: I1eef5d7f5ec5464e0f9a1d7f1130d9ba6dea4557