[skottie] Pass asset IDs to ResourceProvider::loadImageAsset

Also use explicit IDs as keys for the image asset cache.

Change-Id: I359ff026063318ace524d1205b4f0b3e7a6e1d5d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227783
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Isabel Ren <isabelren@google.com>
7 files changed