Add failing tests to known-failures.

The tests were added in O to ensure the PreferenceActivity rewrite was
done correctly. However they tend to be quite flaky. This time the
issue is that button element has a longer hover effect which makes it
ruin my awesome screenshots. I need to redesign the tests completely.

Bug: b/62368423
Test: N/A
Change-Id: I1c9a54b0529b466f467ad9358c12876214971750
1 file changed