Add lint test property

Some libraries are only used for tests, but
are not test module types. These modules get warnings
about @VisibleForTesting usages when they really
shouldn't. Expose a test flag that module authors
can use to make lint treat a module as test code.

Bug: 235339747
Test: Manually tested applying it to SystemUI-tests
Change-Id: I1356749a669dc80a7725605d7159da27c9a211b4