Lockscreen notifications: Add "show all notifications" navbar button

Conflicts:
	packages/Keyguard/src/com/android/keyguard/KeyguardViewMediator.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarTransitions.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java