SystemUI: Add double tap to sleep gesture

Change-Id: I4c54718e8ec8774677d13917b3a7ffa3b4b18cb5
SystemUI: double tap to sleep improvements

* Make it more reliable
* Add it to keyguard
* Add a content observer to not always query Settings.System on every
touch event

Change-Id: I292c4d9d9f810843590b7a9ec6e15b99ac44009d
Signed-off-by: Roman Birg <roman@cyngn.com>