Update simple_sdf sample to use uniform-by-name accessor

[andywolff]

The newer, more convenient method getUniformVec2 is now available on stable, so let's use that in the simple_sdf sample

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

[gaaclarke]

LGTM, but I think we need to wait a couple of days for 3.44 to land on stable before landing this. You can double check against stable, but I don't think it should work.

[andywolff]

LGTM, but I think we need to wait a couple of days for 3.44 to land on stable before landing this. You can double check against stable, but I don't think it should work.

Confirmed this does not render in chrome on stable (3.41) but does work on 3.44. I'll wait for that before merging.