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>
Signed-off-by: Chet Kener <Cl3Kener@gmail.com>