base

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 ab55e524b89c13082193940a5bf8480a2c2522d9)

Fixes bug 13225149.

Change-Id: Ib6f33cbcfb2f60d8957a9184729ecdec80c6a236