Show the keyguard if unsecure window on top

When traversing windows save the fact that windows above the topmost
fullscreen window do not hide the lockscreen. If there is such a
window don't show it.

(cherry picked from commit ab55e524)

Fixes bug 13225149.

Change-Id: Ib6f33cbcfb2f60d8957a9184729ecdec80c6a236