Copy junit-runner files into test-runner.

The android.test.runner target forms part of the Android API and
so must maintain backwards compatibility. The junit classes that
belong in there are copied here to ensure that they do not
change when external/junit is upgraded.

Bug: 30188076
Test: make checkbuild and checked android.test.runner contents
Change-Id: I947144c47ae1c3eb361a43c39bdd03dc11b9575f