This PR is to make AppBarTheme conform to Flutter Material's conventions for component themes:

• Added a AppBarThemeData class which defines overrides for the defaults for AppBar properties.
• Added AppBarTheme constructor parameters: AppBarThemeData? data and Widget? child. This is now the preferred way to configure a AppBarTheme:

AppBarTheme(
      data: AppBarThemeData(),
      child: Scaffold(
        appBar: AppBar(
          title: xxx,
          leading: xxx,
          actions: xxx,
     ),
),

These two properties are made nullable to not break existing apps which has customized ThemeData.appBarTheme.

• Update AppBarTheme to be an InheritedWidget subclass.
• Changed the type of component theme defaults from AppBarTheme to AppBarThemeData.
• Add new tests for AppBarThemeData and update the existing AppBarTheme tests.
• This also temporarily changes AppBarThemeData to Object? in ThemeData class, to bypass g3 tests, following Normalize BottomAppBarTheme #168586 (comment).
• Addresses the "theme normalization" sub-project within ☂️ Material Theme System Updates #91772.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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