Bug 805586 - Support "Swipe down" gesture to hide the keyboard.

1. To fix the issue that keyboard may ignore all touch event after swipe
down.
2. Also change from asyncStorage to mozSettings to keep ftu.enabled, so
that we could disable it when doing automated testing of keyboard.
Merge pull request #10959 from RudyLu/keyboard/swipe_hide_keyboard_3

Bug 805586 - support "Swipe down" to hide the keyboard. r=janjongboom