Mark SystemUI-tests as test code for lint

Lint warns on usages of @VisibleForTesting methods from
non-test code. Since SystemUI-tests is just a regular
android_library instead of an android_test, it was warning
there. Mark it as a test module so lint knows not to warn.

Fixes: 235339747
Test: Presubmits
Change-Id: Ibf8e94c4f9582740e60e285cc918b2976c1fc19a