Don't try to remove Views that about to be removed with the root View

Summary:
We keep a list of FlatShadowNodes that mount to Views that we want to delete, and only flush it at the end of an update cycle. This results in a situation where a root view is being removed before children are removed, which results in a crash in NativeViewHierarchyManager because a View that we are trying to remove no longer exists. There are a few approaches to fix the issue:
a) make a check if a View exists before removing it. While works, it removes a bug protection when we erroneously trying to remove a View that no longer exists (such as this). I'd prefer to keep the check in place
b) flush the views-to-drop queue. This works, but does some extra work in UI thread (namely, removing Views that would be removed anyway)
c) trim the views-to-drop queue to remove any Views that will be removed anyway. This does a tiny bit of extra work in BG thread, but less work in UI thread.

This diff implements option c).

Reviewed By: ahmedre

Differential Revision: D2990105

ReactAndroid/src/main/java/com/facebook/react/flat/FlatUIImplementation.java

@@ -389,6 +389,12 @@ public class FlatUIImplementation extends UIImplementation {
     mStateBuilder.applyUpdates(eventDispatcher, (FlatRootShadowNode) cssNode);
   }
 
+  @Override
+  public void removeRootView(int rootViewTag) {
+    mStateBuilder.removeRootView(rootViewTag);
+    super.removeRootView(rootViewTag);
+  }
+
   @Override
   public void setJSResponder(int possiblyVirtualReactTag, boolean blockNativeResponder) {
     ReactShadowNode node = resolveShadowNode(possiblyVirtualReactTag);