Show IME over keyguard if IME target is showing

IME was being hidden by keyguard even if the window it was attached
to was showing. This change shows the IME if the window it is
attached to is hiding the keyguard.

Also fix an NPE that was crashing systemui.

Fixes bug 16457525.

Change-Id: If587eefa7b1e0ae26713906a76c0766ebaae69a6