fix(modal): fix backdrop animation for sheets with off-center backdropBreakpoint

[averyjohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: Resolves part of #23923

The backdrop animation is incorrect when dragging a sheet modal with a backdropBreakpoint that isn't 0, 1, or 0.5. (See recordings)

What is the new behavior?

Backdrop now animates as expected.

Does this introduce a breaking change?

• Yes
• No

Other information

Sheet modal shown below with { backdropBreakpoint: 0.3, initialBreakpoint: 0.3, breakpoints: [0.3, 0.5, 0.7, 1] }.

Before: https://user-images.githubusercontent.com/90629384/136992903-c32b63b5-24c3-43c6-b51c-727c68a22ae1.mp4
After: https://user-images.githubusercontent.com/90629384/136991453-48e3832f-90f6-4899-a4ba-1fc3c0c4527b.mp4