base

Make magnification ignore invisible wallpaper.

Magnification was assuming that all windows for which
isOnScreen() returned true were visible, but that is not
true for at least wallpapers that have been made invisible.
Adding a check for visibility in addition to on screen.

Bug: 7215285
Change-Id: Iadea37c9eec981f1d68c5a42959debb58cbab264