Remove overly tight constraint on drawn Keyguard.

The requirement that a window that is invisible will not be drawn is
incorrect. In particular the test fails before a surface has even been
added (mHasSurface == false) or shown (mPolicyVisibility == false).
This was causing the screen to turn on before Keyguard had been drawn
and exposing surfaces that would have normally remained hidden.

Also, don't pass null into KeyguardServiceDelegate.onShown() or we
will immediately turn the screen on before Keyguard is drawn.

Fixes bug 11062635.

Change-Id: I964c7ef186d0a94678020b9c27ca6b79e5433350