Make shader warm-up async so it can handle image (#28687)

## Description

This moves another 15-20ms from the animation jank of one of our important client to the startup latency. Unfortunately, this is probably not captured in our current benchmarks (presumably some other bottlenecks overshadow this shader compilation in the worst_frame benchmark). Considering that drawing images is such a common operation, maybe we should add one in the future to benchmark this.

We need this PR to land soon for our client because this changes the API to return Future.

## Related Issues

https://github.com/flutter/flutter/issues/813