[material_ui] Default gen_defaults color helper prefix

[QuncCccccc]

Work toward flutter/flutter#187899.

Follow-up to #12846.

While migrating the M3 templates, I noticed that existing templates use _colors as the prefix passed to color and colorWithOpacity. This PR makes _colors the default value for the prefix argument and removes the now-redundant explicit arguments from existing templates. Explicit custom prefixes remain supported, and regenerating the enabled templates produces no changes to the checked-in generated defaults.

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Code Review

This pull request refactors several template files under packages/material_ui/tool/gen_defaults/templates/ and their associated tests by making the prefix parameter of the color and colorWithOpacity helper methods optional with a default value of '_colors'. This allows removing the explicit '_colors' argument from numerous calls across the templates. The reviewer suggests replacing deprecated MaterialStatePropertyAll with WidgetStatePropertyAll in button_template.dart, utilizing the inherited border helper method instead of explicitly instantiating BorderSide, and simplifying redundant conditional branches in the iconColor resolver.