Implement returnKeyType/returnKeyLabel on Android

Summary:
This PR implements [`returnKeyType`](http://facebook.github.io/react-native/docs/textinput.html#returnkeytype) on Android.

It is implemented with [`EditText.setImeOptions()`](http://developer.android.com/reference/android/widget/TextView.html#setImeOptions(int)) that allows us to specify options on an input, in this case change the return button icon. To be noted that it is also possible to specify a string instead of an icon with [`EditText.setImeActionLabel()`](http://developer.android.com/reference/android/widget/TextView.html#setImeActionLabel(java.lang.CharSequence, int)) while being 2 different things I added both of these behaviors in this PR since it was mostly at the same place/component.

**Problems encountered :**

- All the `ReactEditText`s were set to `IME_ACTION_DONE` by default ([reference](a2157dbbe0/ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactTextInputManager.java (L78))). I'm not sure
Closes https://github.com/facebook/react-native/pull/6905

Differential Revision: D3264755

Pulled By: dmmiller

fb-gh-sync-id: 4a090e31b620a245847c06ba1895cfea02e88d0f
fbshipit-source-id: 4a090e31b620a245847c06ba1895cfea02e88d0f

Examples/UIExplorer/TextInputExample.android.js

@@ -531,4 +531,43 @@ exports.examples = [
       return <TokenizedTextExample />;
     }
   },
+  {
+    title: 'Return key',
+    render: function() {
+      var returnKeyTypes = [
+        'none',
+        'go',
+        'search',
+        'send',
+        'done',
+        'previous',
+        'next',
+      ];
+      var returnKeyLabels = [
+        'Compile',
+        'React Native',
+      ];
+      var examples = returnKeyTypes.map((type) => {
+        return (
+          <TextInput
+            key={type}
+            returnKeyType={type}
+            placeholder={'returnKeyType: ' + type}
+            style={styles.singleLine}
+          />
+        );
+      });
+      var types = returnKeyLabels.map((type) => {
+        return (
+          <TextInput
+            key={type}
+            returnKeyLabel={type}
+            placeholder={'returnKeyLabel: ' + type}
+            style={styles.singleLine}
+          />
+        );
+      });
+      return <View>{examples}{types}</View>;
+    }
+  },
 ];