Get rid of Keyguard visibility modifiers on WindowState

Not needed anymore \o/. Also fixes a flicker when transitioning
between two activities that both set FLAG_LIGHT_STATUS that was
somehow introduced recently.

Test:
- Start an light status bar activity from another activity that
already has light status bar, ensure there is no flicker.
- Open IME and make sure the content gets resized like before.

Change-Id: Ie9c9e1cd40f909c449d36ae436250063af20539e