As pointed out in #79595, the padding used by the new ReorderableListView doesn't scroll property with the list and also has issues when there is no header. This PR will fix both issues.

I have added a couple of new tests to verify that the padding is setup correctly.

Fixes: #79595

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.