frameworks: allow Keyguard package to be defined in overlay

This way, custom keyguards can have their own namespace.

Change-Id: Ia2dfa57652c6f3d3404963897fb2f20a265e5c76
Signed-off-by: Roman Birg <roman@cyngn.com>

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java