[material_ui] Migrate M3 ExpansionTile template to use new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899.
Fixes flutter/flutter#188414.

This PR migrates the M3 ExpansionTile defaults template to use the new gen_defaults in material_ui.
The generated defaults remain unchanged.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request migrates the generation of Material 3 defaults for ExpansionTile to the token-based generator. It extracts the _ExpansionTileDefaultsM3 class into a generated part file, introduces the ExpansionTileTemplateM3 template to generate these defaults from design tokens, and adds corresponding unit tests. The review feedback suggests simplifying the generated template code by removing the unused _theme field and directly initializing _colors.