[canvaskit] Update all demos to use full version from 0.25.0

Also add some explanation to the image_sampling demo.

Change-Id: Id20fe4d47c45b6a6b27e227871458bb862ec83d8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378936
Reviewed-by: Mike Reed <reed@google.com>
9 files changed