Initial support for notification panel access in keyguard.

The special touch target and fancy animations (see bug
5052059) aren't there yet; we'll probably need a different
flavor of DISABLE_EXPAND to specify that state. But this is
a start and should get the basic feature into testers'
hands.

Bug: 5052059
Change-Id: I967e4557e467d1762d6566643bb401eb84f94e92