Fix incorrect view flattening when usign a specific not fully transparent color - #51379

…rent color (#51379)

Summary:

Changelog: [General][Fixed] Fix incorrect flattening / non-rendering of views with backgroundColor set to `rgba(255, 255, 255, 127/256)`

Fixes #51378.

## Context 

When testing some unrelated things with Fantom is realized that the color for some text that I wasn't explicitly defining was being set to `rgba(255, 255, 255, 127)`, like here:

https://github.com/facebook/react-native/blob/249a24ac756275eadbe3b4df1ff9c974af1671d2/packages/react-native-fantom/src/__tests__/Fantom-itest.js#L540-L542

When digging a bit more about why, I realized that was actually the value for `UndefinedColor`. When looking a bit deeper, I saw that the value for that constant was being set like this:

```
using Color = int32_t;

namespace HostPlatformColor {
static const facebook::react::Color UndefinedColor =
    std::numeric_limits<facebook::react::Color>::max();
}
```

I'm not sure what the logic could've been here:
- Defining it as a value out of bounds for all valid colors? In this case, it's a 32 bit value so all the range of values are actually valid RGBA colors.
- Defining it as a fully opaque white? Seems dangerous for a default because you wouldn't be able to distinguish a explicitly set white color from a non-set color, relevant if you're seeing a white background color in a view on top of another view with any other background color.

The result of this existing logic was actually setting `UndefinedColor` to `rgba(255, 255, 255, 127)` because the alpha channel is defined in the first bits of the value, and `Color` being a signed int with 32 bits, the largest value is `01111....1`, so extracting the first 8 bits, you get 127.

## Changes

This changes the value set for the `UndefinedColor` constant (which is used, among other things, to determine if a view sets a background color, or otherwise could potentially be flattened).

The new value, instead of white with a 127/256 opacity, is black with 0% opacity (or simply the number 0 in `int32_t`).

Reviewed By: javache

Differential Revision: D74869311