fixes [[Material3] AppBar does not respect `foregroundColor` or `iconTheme` for leading and actions in some cases](https://github.com/flutter/flutter/issues/130485)

### Description

- Fix `Colors.white` not applied in dark mode
- Add regression tests
- make `iconStyle` private for consistency

### Before
![Screenshot 2023-07-14 at 18 40 58](https://github.com/flutter/flutter/assets/48603081/a6caffd6-d9a1-407a-aea7-c30047bfe7c7)

### After
![Screenshot 2023-07-14 at 18 41 04](https://github.com/flutter/flutter/assets/48603081/f864da7a-2ff8-46a4-8927-591e50050502)