This attempts to address two sources of flake:
- A test that playing doesn't continue past the duration flakily fails, at least on Android, with a position a small amount past the duration. This seems like an unexpected library behavior that we wouldn't want to expose to clients, so rather than change the test, this makes the logic that updates the `Value` clamp the position to the duration.
- A test that asset videos play has been restructured to actually wait for the future that should start playing to complete before checking whether it's playing. The test was previously not actually waiting for anything other than animations to complete, and there's no reason the placeholder layout couldn't have completed before the asset loaded. The fact that the test was already disabled for iOS is strong evidence that the flake we are seeing on Android is a problem with the test itself, so hopefully this addresses both platforms.

Fixes https://github.com/flutter/flutter/issues/86915