This PR fixes inconsistency of SelectionArea when scrolling by removing the `_clearSelection` on `longPressUpdate` gesture. This in turn makes the selection UX much better.

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes: https://github.com/flutter/flutter/issues/120892

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*