Flow strict TextInput (#22250)

Summary:
Related to #22100

Enhance TextInput with callback event types.
This is a first draft and I will need more help on this one. Flow checks are successful now but I am not sure types are accurate though.
Moreover I find my separation approach kind of dirty for callback event types.

- All flow tests succeed.

[GENERAL] [ENHANCEMENT] [TextInput.js] - Flow types
[GENERAL] [ENHANCEMENT] [TextInputExample.android.js] - Fixing Flow types
[GENERAL] [ENHANCEMENT] [TextInputExample.ios.js] - Fixing Flow types
[GENERAL] [ENHANCEMENT] [XHRExampleFetch.js] - Fixing Flow types
Pull Request resolved: https://github.com/facebook/react-native/pull/22250

Reviewed By: TheSavior

Differential Revision: D13104820

Pulled By: RSNara

fbshipit-source-id: 3fbb98d0ec2b62be676f71ae1053933d9c78485e

RNTester/js/TextInputExample.android.js

@@ -48,7 +48,8 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
           }
           onContentSizeChange={event =>
             this.updateText(
-              'onContentSizeChange size: ' + event.nativeEvent.contentSize,
+              'onContentSizeChange size: ' +
+                JSON.stringify(event.nativeEvent.contentSize),
             )
           }
           onEndEditing={event =>
@@ -253,10 +254,10 @@ class ToggleDefaultPaddingExample extends React.Component<
 }
 
 type SelectionExampleState = {
-  selection: {
+  selection: $ReadOnly<{|
     start: number,
-    end: number,
-  },
+    end?: number,
+  |}>,
   value: string,
 };