Keep callback reference from being null.

The hide call delays removal of mKeyguardView but the inflate call
occurred before the delay expired. The method inflateKeyguardView
would have removed it except hide() also nulled out mKeyguardView.
Consequently there were two KeyguardHostViews in the view hierarchy.
The findViewById returned the first view which got the callback
assigned. Later that view was removed from the hierarchy and when
the new view was called it had no callback assigned => NPE!

This fix removes the KeyguardHostView from the mKeyguardHost by
referncing the id rather than the null mKeyguardView.

Fixes bug 7250008.

Change-Id: Ic9da3415d72187628f06271c12d5184f9d01059b