Changes drag release logic so that an armed refresh is only canceled if the user has scrolled back up beyond the point where the refresh indicator was armed. (Fixes https://github.com/flutter/flutter/issues/138848.)

This is the minimal change I found could be made to restore something like the behavior that I would expect.

This may still need a bit of work, because it only masks the second issue I mentioned, that releasing a drag can cause the scroll position to be animated back up from the release point. There is actually a bug about that: https://github.com/flutter/flutter/issues/6052. I would like to see that bug fixed too. This PR doesn't address that, but makes it harder to hit that issue.

@Piinks this is a recreation of #139015 (since I couldn't figure out some issue with a git detached branch, so I fixed the PR and I'm re-submitting it). This version includes one line that was somehow accidentally dropped from the original PR. This will hopefully fix the test failures.

However, I don't have a clue how to write a test for a Flutter UI widget. I'll try to figure that out, but also I don't have a lot of time to work on this. I would appreciate at least some user testing to verify that the new behavior is much more intuitive than the old behavior.

- [?] All existing and new tests are passing.