Fix modals when using Nodes

Summary:
Modals were broken in Nodes, because the custom measurement logic for
all the children of the ReactModalShadowNode was not being applied (because we
wrapped it in a NativeViewWrapper). This change adds a custom flat node type
for modals.

Differential Revision: D3499557

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

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.flat;
+
+import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.uimanager.LayoutShadowNode;
+import com.facebook.react.views.modal.ReactModalHostManager;
+
+/* package */ class RCTModalHostManager extends ReactModalHostManager {
+
+  /* package */ RCTModalHostManager(ReactApplicationContext context) {
+    super(context);
+  }
+
+  @Override
+  public LayoutShadowNode createShadowNodeInstance() {
+    return new FlatReactModalShadowNode();
+  }
+
+  @Override
+  public Class<? extends LayoutShadowNode> getShadowNodeClass() {
+    return FlatReactModalShadowNode.class;
+  }
+}