Revert "Frameworks: disable lock screen heads up (1/2)"

This reverts commit ebf2e5ffea8c969a17b5801f5799ae6ee60502b1.

Conflicts:
	packages/Keyguard/src/com/android/keyguard/NotificationHostView.java

Change-Id: I070ca59fe44b0b646ac604f2adb0c6f719f1df11