relativeKeyboardHeight works wrong in some cases

Summary:
relativeKeyboardHeight calculation is wrong when frame.y < keyboardFrame.y

Let's look at an example:

``` jsx
import React, { Component } from 'react';
import {
  KeyboardAvoidingView,
  TextInput,
  AppRegistry,
  Text,
  View
} from 'react-native';

export default class Test extends Component {
  constructor(props, context) {
    super(props, context);
  }

  render() {
    return (
      <View style={{flex: 1, marginTop: 20}}>
          <Text style={{ flex: 1, alignItems: 'center'}}>
            Hi! This is test.
          </Text>

          <KeyboardAvoidingView behavior={'position'} keyboardVerticalOffset={-20} >
            <TextInput
              style={{height: 40, borderColor: 'gray', borderWidth: 1}}
            />
          </KeyboardAvoidingView>
      </View>
    );
  }
}

AppRegistry.registerComponent('Test', () => Test);
```

![rn-relativekb](https://cloud.githubusercontent.com/assets/13254349/17298119/b2192830-5819-11e6-968c-d9e95b6e4aba.gif)

How _relativeKeyboardHeight_ calculates positio
Closes https://github.com/facebook/react-native/pull/9139

Differential Revision: D4168431

Pulled By: hramos

fbshipit-source-id: 9181276355490452071b51b51ed7dc9ab80b2444

Libraries/Components/Keyboard/KeyboardAvoidingView.js

@@ -94,6 +94,9 @@ const KeyboardAvoidingView = React.createClass({
 
     const y1 = Math.max(frame.y, keyboardFrame.screenY - this.props.keyboardVerticalOffset);
     const y2 = Math.min(frame.y + frame.height, keyboardFrame.screenY + keyboardFrame.height - this.props.keyboardVerticalOffset);
+    if (frame.y > keyboardFrame.screenY) {
+      return frame.y + frame.height - keyboardFrame.screenY - this.props.keyboardVerticalOffset;
+    }
     return Math.max(y2 - y1, 0);
   },