Allow amp-list to render from amp-state initially #15647
Description
See #15613 (comment) for some background.
We should allow amp-list to render from amp-state data on load. This can help with showing "stale" data until remote data is available. It would be nice to also prefetch the endpoint so when a user-interaction ( e.g. "refresh" button) switches the src from amp-state to real endpoint, we would have fetched the data already and swap would be quick.