Do not display unsecure windows behind dialogs

If a dialog activity has FLAG_SHOW_WHEN_LOCKED set it will dismiss
the keyguard. Previously this would expose any full screen unsecure
windows behind the dialog. With this fix the dialog is displayed
over the wallpaper.

Fixes bug 15006623.

Change-Id: I85a6713c7647db52211bd0f7280010e859723710