Fix issue with views becoming visible with stale content.

An earlier fix nooop'd invalidate calls on non-visible views. This
caused an issue where changes to the view while it was not visible
(such as changing the text of a TextView) would not get picked up
by the invalidation process.

The fix is to automatically invalidate() a view when it becomes
visible, to account for any changes that may have occurred to its
content while it was not visible.

Change-Id: Ia0f8fb42ca627b5a38f08761fa622b23f2dfa38b