Fix react/renderer ColorTest (#45485)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45485

## Changelog:

the new functions I introduced in https://github.com/facebook/react-native/pull/45365 to read color channel value, like `alphaFromColor`, will return float between 0~255 on iOS and 8bit unsigned ints on other platforms, because of different platform implementation. However that way we cannot assume consistent return value across platforms, which kind of contradicts with RN's cross platform design.

so here I make `alphaFromColor` in Color.h (the cross platform method) to always return uint8_t, while still allow `alphaFromHostPlatformColor` to return different result

[Internal] [Fixed] -

Reviewed By: christophpurrer

Differential Revision: D59826142

fbshipit-source-id: 4401918be29980474bdc8601443ae33155710f22