deleteRootView: use concurrent pattern with mTagToViewState

Summary:
Instead of doing a "containsKey then get", just get the rootViewTag and see if it's non-null. Theoretically, since it's a
concurrent data-structure, it could be removed from the ConcurrentHashMap between "containsKey" returning true and the "get".

This does not fix any known, existing problems.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D25378703

fbshipit-source-id: 62a44e68e4443dac5a557263cc4bb33de9eea993

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

@@ -124,11 +124,12 @@ public class MountingManager {
         });
   }
 
-  /** Delete rootView and all children/ */
+  /** Delete rootView and all children recursively. */
   @UiThread
   public void deleteRootView(int reactRootTag) {
-    if (mTagToViewState.containsKey(reactRootTag)) {
-      dropView(mTagToViewState.get(reactRootTag).mView, true);
+    ViewState rootViewState = mTagToViewState.get(reactRootTag);
+    if (rootViewState != null && rootViewState.mView != null) {
+      dropView(rootViewState.mView, true);
     }
   }