Start using gaia's desktop helper to fix touch events and gain API mocks

By using desktop helper, we will be able to fix touch events and also share API mocks available here:
https://github.com/mozilla-b2g/gaia/tree/master/tools/extensions/desktop-helper/content/data/lib

This patch depends on gaia modification from https://bugzilla.mozilla.org/show_bug.cgi?id=869540.