Fix: Page could trigger Timer cancel on shutdown.

If the user touched a Page while its ending animation was playing, the
Page would try to signal that the Page was still active. This fix tells
the Page that it is no longer 'timed' when it's shutting down.
Fix: ImageCache could cache a photo with bad res.

If the ImageCache cached a photo before the PageManager had a chance to
update the screen resolution value, the photo would be cached with a
tiny resolution. Changed so that the screen resolution update is the
first thing the PageManager does.