Bug 856989 - Use touchstart rather than tap on lockscreen buttons

Fixes [856989](https://bugzilla.mozilla.org/show_bug.cgi?id=856989).

This PR fixes 2 issues:

1. Users drag the item, this PR unlocks ontouchstart rather than tap, which means that the phone will unlock regardless of where the touch ends
2. There is always a delay between touchstart and touchend, so unlocking the phone is notably faster than before (I'd say ~100 ms. on my phone).