make RenderResult constructor public

Summary:
In Signals, we want to be able to reuse `RenderTree`s across generations if the associated layout result and layout signal hasn't changed. This is particularly of use in determining whether to invalidate a collection's cells as will be seen in upcoming diffs

However, none of `RenderResult`s static creator functions allow this because the decision to reuse a previous `RenderTree` is based off of node/model pointer equality which no longer really works in Signals

Hence, we're opening up the constructor of `RenderResult` to allow callers to instantiate a `RenderResult` with a `RenderTree` of their choice.

Reviewed By: pasqualeanatriello

Differential Revision: D68839596

fbshipit-source-id: 52bf736ad135f20c2647b0efa3a2f0f24f6de1c2

litho-rendercore/src/main/java/com/facebook/rendercore/RenderResult.kt

@@ -25,8 +25,7 @@ import com.facebook.rendercore.extensions.RenderCoreExtension
  * computation will make the next computation of a new [RenderResult] more efficient with internal
  * caching.
  */
-class RenderResult<State, RenderContext>
-internal constructor(
+class RenderResult<State, RenderContext>(
     val renderTree: RenderTree,
     val nodeTree: Node<RenderContext>,
     val layoutCacheData: Map<Any, Any?>,