[Android] Temporarily disable some Core tests.

Disable OnFullscreenToggledTest, OnRequestFocusTest and
OpenFileChooserTest.

They have started failing due to
https://codereview.chromium.org/516753002 and
https://code.google.com/p/chromium/issues/detail?id=407643, and fixing
them will likely require getting some patches upstream.

While we do that, let's get the tests back to green to avoid letting
other regressions creep in.

Related to XWALK-2755.