Fabric: Introducing AttributedStringBox

Summary:
The diff implements a new class called `AttributedStringBox` that represents an object storing a shared `AttributedString` *or* a shared pointer to some opaque platform-specific object that can be used as an attributed string. The class serves two main purposes:
- Represent type-erased attributed string entity (which can be platform-specific or platform-independent);
- Represent a container that can be copied with constant complexity.

Why? Several reasons:
- Sometimes it makes sense to keep an attributed string as a shared resource. This way we don't need to pay for expensive copying and we also implement a copy-on-write semantics on top of that if needed.
- We need to extend a TextLayoutMeasure API to support measuring some platform-specific attributed string implementation to remove the necessity of converting a string back and forth between representations. That's especially important for TextInput because we will need to measure that very efficiently (and the source of measuring, in this case, is a platform attributed string).

In other words, we need something to store inside TextInputState to measure and update very efficiently. The source of this data might be a native TextInput control or a data from React, to represent that kinda object we need this data structure (and interfaces that deal with it).

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D18670793

fbshipit-source-id: bc0164f801f28642f7c6da340af12acf33b85d24

ReactCommon/fabric/textlayoutmanager/platform/android/TextLayoutManager.cpp

@@ -23,9 +23,11 @@ void *TextLayoutManager::getNativeTextLayoutManager() const {
 }
 
 Size TextLayoutManager::measure(
-    AttributedString attributedString,
+    AttributedStringBox attributedStringBox,
     ParagraphAttributes paragraphAttributes,
     LayoutConstraints layoutConstraints) const {
+  auto &attributedString = attributedStringBox.getValue();
+
   return measureCache_.get(
       MeasureCacheKey{attributedString, paragraphAttributes, layoutConstraints},
       [&](MeasureCacheKey const &key) {