Relands #118782 with some changes.

The original PR added a behavior change--when trying to load an asset and we don't find its key in the asset manifest file, we would throw an error (code). The idea was that this would provide a more useful error message than the one that would get shown when the image inevitably fails to load (code). However, this caused the errorBuilder functionality of Image to not work in the case of a missing asset (this was revealed by an internal test breakage). Fixing this appears non-trivial, so I think it's better to scope this out of this PR and perhaps look at it again later.

This also makes a small update to the recently-added AssetManifest.getAssetVariants API (still in beta). When given a key that is not found within the asset manifest file, the method will return null instead of throwing an error. Throwing an error is a bit too opinionated for an API that's just meant to provide read-access to the asset manifest file. The tradeoff is that callers will have to do null checking/assertions.
Let me know if you feel strongly that this should be its own PR.

Pre-launch Checklist

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