[material_ui] Migrate AppBar M3 template to use new gen_defaults script (#12707)

Work towards https://github.com/flutter/flutter/issues/191088

**Ports over https://github.com/flutter/packages/pull/12122 which landed on the `m3e_migration` feature branch.**

### Original PR description

Fixes https://github.com/flutter/flutter/issues/188397

Initially gen_defaults script and its templates were used to handle JSON tokens. After we fix the token pipeline and make it possible to get M3E tokens, we use Dart constants and re-created the gen_defaults in material_ui. After decoupling, the old gen_defaults will be deprecated soon.

This PR is to:

create a template for M3 AppBar so we can use new script to generate M3 defaults.
move the generated code to generated/ folder so all M3 and M3E generated code will be in the same location.

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