Implement addUIBlock to allow third party to resolve a view

Summary:
This PR follows the work started in #6431 but instead of implementing the snapshot for Android, will just allow that feature to be implemented by a third party library.

The missing feature is the ability to resolve a View for a given tag integer. which is now possible with a `addUIBlock` on the `UIManagerModule` method where you give a `UIBlock` instance (a new class) that implements a `execute(NativeViewHierarchyManager)` function.

This is already possible in iOS API. a third party can use the `addUIBlock` too.
I have kept the name `addUIBlock` so it's the same as in the [iOS' API](https://github.com/facebook/react-native/search?q=addUIBlock&type=Code&utf8=%E2%9C%93).

 ---

With this PR a third party lib can now do...

```java
UIManagerModule uiManager = reactContext.getNativeModule(UIManagerModule.class);
uiManager.addUIBlock(new UIBlock() {
  public void execute (NativeViewHierarchyManager nvhm) {
    View view = nvhm.resolveView(tag);
    ...do something with view... like... screenshot t
Closes https://github.com/facebook/react-native/pull/8217

Differential Revision: D3469311

Pulled By: astreet

fbshipit-source-id: bb56ecc7e8936299337af47ca8114875ee1fd2b0

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

@@ -83,7 +83,7 @@ public class NativeViewHierarchyManager {
     mRootTags = new SparseBooleanArray();
   }
 
-  protected final View resolveView(int tag) {
+  public final View resolveView(int tag) {
     View view = mTagsToViews.get(tag);
     if (view == null) {
       throw new IllegalViewOperationException("Trying to resolve view with tag " + tag
@@ -92,7 +92,7 @@ public class NativeViewHierarchyManager {
     return view;
   }
 
-  protected final ViewManager resolveViewManager(int tag) {
+  public final ViewManager resolveViewManager(int tag) {
     ViewManager viewManager = mTagsToViewManagers.get(tag);
     if (viewManager == null) {
       throw new IllegalViewOperationException("ViewManager for tag " + tag + " could not be found");