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.ios.js

@@ -71,7 +71,7 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
               'onSelectionChange range: ' +
                 event.nativeEvent.selection.start +
                 ',' +
-                event.nativeEvent.selection.end,
+                (event.nativeEvent.selection.end || ''),
             )
           }
           onKeyPress={event => {
@@ -348,10 +348,10 @@ class BlurOnSubmitExample extends React.Component<{}> {
 }
 
 type SelectionExampleState = {
-  selection: {|
+  selection: $ReadOnly<{|
     start: number,
     end?: number,
-  |},
+  |}>,
   value: string,
 };