ExpandIcon Custom Colors

[shihaohong]

Description

• Adds custom icon color, expanded icon color, and disabled icon colors to the ExpandIcon widget. As part of this change, ExpandIcon now uses Colors.white60 instead of Colors.white54 when dark theme is used.
• Updates the dark mode ThemeData.disabledColor value to match the Material Design specification. See Update default colors for enabled/disabled text/iconography #33149
• Fixes the Colors.white30's documented opacity value, which was incorrectly stating it was 32% opacity instead of 30%.

This takes over #31053

Related Issues

#18992
#21328

Tests

I added the following tests:

• Tests to verify default light/dark mode ExpandIcon's color and its disabled color
• Tests to verify ExpandIcon takes custom icon color, expanded icon color, and disabled icon color

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• 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 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

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes) - https://groups.google.com/forum/#!topic/flutter-announce/22w45SnuZXM
• No, this is not a breaking change.

[HansMuller]

This change is likely to have an impact on many apps and tests. Even though it's a tiny change, it would be good to move it to a separate PR.

[shihaohong]

That makes sense. I'll make the color update into a separate PR

[shihaohong]

Tracking here: #33149