Remove overscroll effect from AppBars

[micahmo]

Bug Description

This is a super small issue, which I am mainly creating so I can track it and take it off my to-do list. 😊

After switching to SliverAppBars, I noticed that that the Android overscroll effect now applies to the contents of the AppBar as well. This is odd, mainly because the AppBar is not part of the scrollable content. You would expect the overflow to be applied only to the contents beneath the AppBar.

You can see a demo of the issue in flutter/flutter#104437. There are a couple workarounds, but for now I'd rather just follow that issue until there's a good "fix" we can apply.

Note that iOS does not have this problem.

Expected Behaviour

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Steps to Reproduce

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Additional Context

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App Version

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Device

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OS

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