Adjust padding for Cupertino sheet content (#162481)

Fixes #162215 and partially supports #162181.

The Cupertino sheet had two issues layout: It still had MediaQuery
padding at the top of the screen as if there was safe area content to
avoid and some of the sheet was permanently offscreen. This caused a
FloatingActionButton to be clipped, but if you put a scrolling widget in
the sheet with text at the very bottom, it was impossible to scroll the
text into view. This PR removes the top padding, and changes the bottom
padding to equal the amount offscreen.

Before:
<img width="396" alt="Screenshot 2025-01-30 at 11 21 36 AM"
src="https://github.com/user-attachments/assets/4e27db47-8d54-44c7-8cba-58790b88fef3"
/>

After:
<img width="396" alt="Screenshot 2025-01-30 at 11 19 54 AM"
src="https://github.com/user-attachments/assets/68f056f2-7731-4a56-8124-187d7efae020"
/>


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