Move to{HashMap,ArrayList} up to Readable{Map,Array}

Summary:
NativeReadable{Map,Array} classes have convenient to{HashMap,ArrayList}
methods that make it easier to interoperate with existing Java code
from within a ReactNative application (e.g., Native Module) ...

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- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.
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NativeReadable{Map,Array} classes have convenient to{HashMap,ArrayList}
methods that make it easier to interoperate with existing Java code
from within a ReactNative application (e.g., Native Module)

These changes make these same methods available to any code using
the Readable{Map,Array} interfaces, instead of forcing consumers to
cast their generic instances into the NativeReadable* equivalents

Moving this methods up to the interfaces also makes it easier to
write unit tests for Native Modules - using the JavaOnly{Map,Array}
implementations of Readable{Map,Array} - while still relying on the
to{HashMap,ArrayList} methods

* Write a native module that receives a JSON object as `ReadableMap` and a JSON array `ReadableArray`.
* Print out the result of `toHashMap` and `toArrayList`.
* Make sure `NativeReadable{Map,Array}` works:
  * Call the native module's method from JavaScript, passing an `Object` and an `Array`.
  * Compare the printed values with the passed content.
* Make sure `JavaOnly{Map,Array}` works:
  * Call the native module's method from the Java code and pass a `JavaOnlyMap` and a `JavaOnlyArray`.
  * Compare the printed values with the passed content.

**Please advise if there is an automated test suite where I could add a case for this.**
Closes https://github.com/facebook/react-native/pull/14072

Differential Revision: D5123120

Pulled By: javache

fbshipit-source-id: 343f4396b99e03ecaf47993db6955d7932152f77

ReactAndroid/src/main/java/com/facebook/react/bridge/ReadableArray.java

@@ -9,6 +9,8 @@
 
 package com.facebook.react.bridge;
 
+import java.util.ArrayList;
+
 /**
  * Interface for an array that allows typed access to its members. Used to pass parameters from JS
  * to Java.
@@ -25,4 +27,6 @@ public interface ReadableArray {
   ReadableMap getMap(int index);
   Dynamic getDynamic(int index);
   ReadableType getType(int index);
+  ArrayList<Object> toArrayList();
+
 }