[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.

BUG=XWALK-2755
Merge pull request #2523 from rakuco/temporarily-disable-some-android-tests

[Android] Temporarily disable some Core tests.