Fix crash introduced by D21735940 in T67525923

Summary:
We cannot call `parentView.getChildCount()` directly, we must get the child count through the ViewManager:

A simple `Log.e` call shows:

```
MountingManager: parentView.getChildCount(): 0 // viewGroupManager.getChildCount(parentView): 7
```

This difference does not occur for ALL views, but it occurs for... enough that this will crash on basically ~every screen, at least on navigation when all views are removed.

Theory about why this is happening: some ViewGroup types compute childCount differently, especially if we do some sort of custom child management for some view type? By coercing to `T` which the ViewManager does, we call getChildCount on the correct class type. This is just a hypothesis, though. But the failing views are all `View`s and it does look like `ReactClippingViewManager` has custom `getChildCount` logic, so that's likely the answer.

There's no such thing as an easy diff!

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D21750097

fbshipit-source-id: 3d87d8f629a0c12101658050e57e09242dfc2a8c

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountingManager.java

@@ -242,7 +242,9 @@ public class MountingManager {
       throw new IllegalStateException("Unable to find view for tag " + parentTag);
     }
 
-    if (parentView.getChildCount() <= index) {
+    ViewGroupManager<ViewGroup> viewGroupManager = getViewGroupManager(viewState);
+
+    if (viewGroupManager.getChildCount(parentView) <= index) {
       throw new IllegalStateException(
           "Cannot remove child at index "
               + index
@@ -253,7 +255,7 @@ public class MountingManager {
               + " children in parent");
     }
 
-    getViewGroupManager(viewState).removeViewAt(parentView, index);
+    viewGroupManager.removeViewAt(parentView, index);
   }
 
   @UiThread