[material_ui] Update Material tokens to version 38.0.15

[elliette]

Contains changes introduced by the following CLs:

• cl/944170221
• cl/953009409
• cl/952902672

As part of the changes:

• The color files are now explicit in their brightness, i.e. color_light and color_dark instead of color and color_dark
• If a color value for medium/high contrast matches the value for the default contrast, than we don't include it in the _contrast.dart file. We only include values that are different than the default.

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[gemini-code-assist]

Code Review

This pull request updates the generated default token data files to version 38.1.2. It replaces inline TypescaleStruct definitions with references to predefined TokenTypescale constants across multiple component files, renames TokenColor to TokenColorLight, and removes several color tokens from contrast-specific color files. Additionally, several banner and scrim token files have been deleted. As there are no review comments, no further feedback is provided.

[elliette]

Part of this change includes explicitly including brightness for the color files, so the default (which was light) is now explicitly called color_light

[elliette]

We don't include background here because it's identical to the TokenColorLight.background (ditto for the other removed values)

[QuncCccccc]

The changes on removing non-expressive components and colors look good to me! Should we wait until this reference issue is fixed in b/538295526? Or we can also land it for now and add //TODO when we use these tokens