Control interaction prompt appears too quickly in most cases

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We have a visual and audible (to screen readers) interaction prompt that appears after a fixed time (currently 3 seconds) of idleness starting when <model-viewer> is focused. In many cases, <model-viewer> will be focused as soon as it appears in the document. In this case, we begin measuring idleness even though the model has not finished loading. The result of this is that the visual prompt might appear before or near to when the model has appeared to the user.

We should fix this by only measuring idleness once the model has finished loading.