Follow up to #113212

There are some inconsistencies in how we set and compare the type of scroll notification in the GlowingOverscrollIndicator and the StretchingOverscrollIndicator in each's respective _handleScrollNotification method.

There should be a case where this type check creates a bug, but we haven't identified it yet. Once we can, we can write a test to include with the patch. See more comments and context in #113212.