Add support for UnsafeObject to allow using Flow strict in native modules

Summary:
This new type will be valid in Flow strict mode and can be used by native modules and components to replace `Object`, with the same semantics.

This unblocks the migration of the most modules in the React Native package to Flow strict.

Changelog: [Internal] Add UnsafeObject type compatible with Flow strict mode to use in native modules and components

Reviewed By: RSNara

Differential Revision: D25540631

fbshipit-source-id: 60b80bbc84a53aecc747e3a1799cdf551e1859cd

ReactCommon/react/nativemodule/samples/platform/android/SampleTurboModule.java

@@ -109,6 +109,16 @@ public class SampleTurboModule extends NativeSampleTurboModuleSpec {
     return map;
   }
 
+  @DoNotStrip
+  @Override
+  @SuppressWarnings("unused")
+  public WritableMap getUnsafeObject(ReadableMap arg) {
+    WritableNativeMap map = new WritableNativeMap();
+    map.merge(arg);
+    log("getUnsafeObject", arg, map);
+    return map;
+  }
+
   @DoNotStrip
   @SuppressWarnings("unused")
   @Override