Core React/Litho support and Java codegen

Summary:
Core React/Litho support and Java codegen. View updating still in progress.

Generated ViewManager code:

```
// Copyright 2004-present Facebook. All Rights Reserved.
//
// Autogenerated by ComponentsReactNativeSupportProcessor
package com.facebook.catalyst.samples.componentsembedding;

import com.facebook.litho.Component;
import com.facebook.litho.ComponentContext;
import com.facebook.litho.reactnative.ComponentsShadowNode;
import com.facebook.litho.reactnative.ComponentsViewManager;
import com.facebook.litho.reactnative.ReactLithoView;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.module.annotations.ReactModule;
import com.facebook.react.uimanager.ReactStylesDiffMap;
import com.facebook.react.uimanager.StateWrapper;
import com.facebook.react.uimanager.ThemedReactContext;
import java.util.HashMap;
import java.util.Map;

ReactModule(
    name = "SampleComponent"
)
public class GeneratedSampleComponentViewManager extends ComponentsViewManager {
  Override
  public String getName() {
    return "SampleComponent";
  }

  Override
  public ReactLithoView createViewInstanceWithProps(ThemedReactContext context,
      ReactStylesDiffMap props) {
    Map<String, Object> propsMap;
    if (props != null && props.toMap() != null) {
      propsMap = props.toMap();
    } else {
      // Non-Fabric will always follow this path, initial props are never provided;
      return ReactLithoView.create(context, null, null);
    }
    Component component = createComponentInstanceWithPropsMap(new ComponentContext(context), propsMap);
    return ReactLithoView.create(context, component, propsMap);
  }

  Override
  public Component createComponentInstanceWithPropsMap(ComponentContext context,
      Map<String, Object> propsMap) {
    SampleComponent.Builder componentBuilder = SampleComponent.create(context);
    setComponentBuilderPropsWithMap(componentBuilder, propsMap);
    return componentBuilder.build();
  }

  Override
  public void updateProperties(ReactLithoView view, ReactStylesDiffMap props) {
    super.updateProperties(view, props);
    Map<String, Object> propsMap = props.toMap();
    Map<String, Object> oldProps = view.getProps();
    if (oldProps == null) {
      // Non-Fabric RN will always terminate here; we do not store props, nor do we need to.;
      // Prop updating happens incrementally in non-Fabric RN.;
      return;
    }
    Map<String, Object> mergedProps = new HashMap<>(oldProps);
    mergedProps.putAll(propsMap);
    SampleComponent.Builder componentBuilder = SampleComponent.create(new ComponentContext(view.getComponentContext()));
    setComponentBuilderPropsWithMap(componentBuilder, mergedProps);
    view.setProps(mergedProps);
    view.setComponent(componentBuilder.build());
  }

  private void setComponentBuilderPropsWithMap(SampleComponent.Builder componentBuilder,
      Map<String, Object> propsMap) {
    if (propsMap != null && propsMap.containsKey("title")) {
      componentBuilder.title((((String)propsMap.getOrDefault("title", null))));
    }
    if (propsMap != null && propsMap.containsKey("imageUri")) {
      componentBuilder.imageUri(android.net.Uri.parse(((String)propsMap.getOrDefault("imageUri", null))));
    }
  }

  Override
  public GeneratedSampleComponentShadowNode createShadowNodeInstance(ReactApplicationContext context) {
    return new GeneratedSampleComponentShadowNode();
  }

  Override
  public Class<? extends ComponentsShadowNode> getShadowNodeClass() {
    return GeneratedSampleComponentShadowNode.class;
  }

  Override
  public void updateState(ReactLithoView view, StateWrapper stateWrapper) {
    view.setStateWrapper(stateWrapper);
  }
}
```

Reviewed By: shergin

Differential Revision: D14846423

fbshipit-source-id: 4eeeb991f7e32c0cec5e9307d6175b81c8fd034e

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

@@ -7,6 +7,8 @@
 
 package com.facebook.react.uimanager;
 
+import java.util.HashMap;
+import java.util.Map;
 import javax.annotation.Nullable;
 
 import android.view.View;
@@ -41,6 +43,10 @@ public class ReactStylesDiffMap {
     mBackingMap = props;
   }
 
+  public Map<String, Object> toMap () {
+    return mBackingMap.toHashMap();
+  }
+
   public boolean hasKey(String name) {
     return mBackingMap.hasKey(name);
   }

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

@@ -35,7 +35,14 @@ import javax.annotation.Nullable;
 public abstract class ViewManager<T extends View, C extends ReactShadowNode>
   extends BaseJavaModule {
 
-  public final void updateProperties(@Nonnull T viewToUpdate, ReactStylesDiffMap props) {
+  /**
+   * For the vast majority of ViewManagers, you will not need to override this. Only
+   * override this if you really know what you're doing and have a very unique use-case.
+   *
+   * @param viewToUpdate
+   * @param props
+   */
+  public void updateProperties(@Nonnull T viewToUpdate, ReactStylesDiffMap props) {
     ViewManagerPropertyUpdater.updateProps(this, viewToUpdate, props);
     onAfterUpdateTransaction(viewToUpdate);
   }