snackBar should paint above the bottomSheet

[xu-baolin]

As title.
If the bottomsheet takes all horizontal space (most do), the snackbar is invisible to the user.

Fixes #63254
Close #19547

Test coming soon.

[flutter-dashboard]

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[xu-baolin]

@Piinks Hi,
Do you have any suggestions about the test case? Can I only use the Goden test for this issue?

[Piinks]

@Piinks Hi,
Do you have any suggestions about the test case? Can I only use the Goden test for this issue?

A golden file test SGTM!

[skia-gold]

Gold has detected about 1 untriaged digest(s) on patchset 3.
View them at https://flutter-gold.skia.org/cl/github/73558

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Changes reported for pull request #73558 at sha fa3f901

[Piinks]

This is a great fix @xu-baolin thank you!