Close keyguard user switcher on tap outside

Also closes when tapping the current user.
Also fixes a bug, where the background alpha was
not properly initialized. And while we're at it,
fixes a bug where the user switcher was closable
in simple mode by extending quick settings.

Bug: 17691134
Change-Id: I5444fe42a8a2887ab012ef6cbdcc57ba81a8c1c2