WIP: Update DialogTheme to match the other theme patterns

[darrenaustin]

Description

As part of the effort to update the Themes in the Material library easier to use and understand, this PR updates the DialogTheme class to match the updated design.

The DialogTheme was unfortunately one of the early Themes and doesn't follow the current design. The DialogTheme class isn't a widget and is actually just the data class that a theme would normally use. To fix this we have:

• Renamed DialogTheme to DialogThemeData.
• Provided a new DialogTheme class that is an inherited widget that can be used to override the dialog properties for a widget subtree.
• Changed the default shape's border radius from 2.0 to 4.0 to match the current Material specification.

These are breaking changes so existing apps using DialogTheme will need to be updated (migration guide coming soon).

Tests

The current tests in the material library have been updated to accommodate these changes as well as fully test the new functionality.

Checklist

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them?

• Yes, this is a breaking change.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: Replace with a link to your migration guide