[canvaskit] Ignore image paths in Skottie asset keys

The name should be sufficient.

Change-Id: I451ae4a0f8d0f0864f07abc0405b591a6e2ca769
Reviewed-on: https://skia-review.googlesource.com/c/196506
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>