Refactor MountingManager into MountingManager + SurfaceMountingManager

Summary:
This refactors MountingManager into a minimal API that shims into a more fully-featured SurfaceMountingManager. The SurfaceMountingManager keeps track of surface start/stop, surface ID, and surface Context.

This solves a number of issues around (1) race conditions around StopSurface/StartSurface, (2) memory management of Views, (3)

Concrete improvements:

1. Simpler to reason about race conditions around StopSurface/StartSurface.
2. 1:1 relationship between SurfaceId and all Views/tags.
3. When surface is stopped, all descendent Views can be GC'd immediately.
4. Fixed separation of concerns and leaky abstractions: surfaceId/rootTag and Surface Context are now stored and manipulated *only* in SurfaceMountingManager.
5. Simpler StopSurface flow: we simply remove references to all Views, and the Fragment (outside of the scope of this code) removes the RootView. This will trigger GC and we do ~0 work. Previously, we ran a REMOVE and DELETE instruction and kept track of each View in a HashMap. Now we can simply delete the map and move on.

The caveat: NativeAnimated (or other native modules that go through UIManager). APIs like `updateProps` currently uses only the ReactTag and does not store SurfaceId. This is a good argument for moving away from ReactTag, at least in its current incarnation, but: for now this requires that you do a lookup of a ReactTag across N surfaces (worst-case) to determine which Surface a ReactTag is in.

So, to summarize, the "con" of this approach is that now `getSurfaceManagerForViewEnforced` could be slower. It is used in:
* NativeAnimatedModule calls `updateProps` through UIManager
* FabricEventEmitter calls `receiveEvent` on FabricUIManager directly
* On audit, I could find zero native callsites to `sendAccessibilityEvent` through UIManager

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D26000781

fbshipit-source-id: 386ae40c4333f8c584e05818c404868dbee6ce73

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -724,8 +724,7 @@ public class UIManagerModule extends ReactContextBaseJavaModule
   @ReactMethod
   public void dispatchViewManagerCommand(
       int reactTag, Dynamic commandId, @Nullable ReadableArray commandArgs) {
-    // TODO: this is a temporary approach to support ViewManagerCommands in Fabric until
-    // the dispatchViewManagerCommand() method is supported by Fabric JS API.
+    // Fabric dispatchCommands should go through the JSI API - this will crash in Fabric.
     @Nullable
     UIManager uiManager =
         UIManagerHelper.getUIManager(