Refactor more logic from radio and cupertino radio (#167764)

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This is in prepare for https://github.com/flutter/flutter/pull/167363

The goal for this refactoring is to make sure the new RadioGroup widget
can be used for both CupertinoRadio and Radio, and thus the widget has
to be in widget layer to avoid duplicating code.

Currently CupertinoRadio and Radio shares ToggleableStateMixin in widget
layer. but the mixin itself doesn't contains enough data to implement
radioGroup. Therefore, I moved more share logic from CupertinoRadio and
Radio to widget layer and created a RadioBase widget.

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