[canvaskit] Remove references to particles.skia.org from demos

We should encourage users to use a real CDN, not a random build of ours.

This also fixes a few broken demos.

Change-Id: I3b607665744b5f481bca0b9b3072aaa25b3d2c57
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/371221
Reviewed-by: Kevin Lubick <kjlubick@google.com>
9 files changed