Eliminate static const UndefinedColor from SharedColor (#41364)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41364

Initializing UndefinedColor on iOS and Android is trivial because the platform color is an int32_t. On platforms where the HostPlatformColor header defines a color as a struct (e.g., Windows) it is less trivial to compose a constexpr for the undefined color representation to initialize this static const value with.

As it turns out, UndefinedColor is only used for operator bool in SharedColor, so it's reasonably safe to remove this "public" API (also good to limit the surface of SharedColor).

## Changelog:
[Internal]

Reviewed By: sammy-SC

Differential Revision: D51073395

fbshipit-source-id: 375e43aa9a30d394d35ce2946224563738d8973c

packages/react-native/ReactCommon/react/renderer/graphics/Color.h

@@ -25,9 +25,7 @@ namespace facebook::react {
  */
 class SharedColor {
  public:
-  static const Color UndefinedColor = HostPlatformColor::UndefinedColor;
-
-  SharedColor() : color_(UndefinedColor) {}
+  SharedColor() : color_(HostPlatformColor::UndefinedColor) {}
 
   SharedColor(Color color) : color_(color) {}
 
@@ -44,7 +42,7 @@ class SharedColor {
   }
 
   operator bool() const {
-    return color_ != UndefinedColor;
+    return color_ != HostPlatformColor::UndefinedColor;
   }
 
  private: