Stop expose Image.resizeMode

Summary:
Exposing this enum is essentially useless and at worst is a runtime cost that isn't necessary by just using the string.

The value of this enum, as far as I understand it, is to enforce that only valid options are used. We can enforce this at build time with Flow.

I was able to migrate our codebase with a few Find and Replace for things like

```
resizeMode={Image.resizeMode.contain}
```

Reviewed By: yungsters

Differential Revision: D7983982

fbshipit-source-id: ddd7024023f8d2f01aad1fff6c8103983a1bec1a

Author: elicwhite

Libraries/Image/Image.android.js

@@ -10,7 +10,6 @@
 
 'use strict';
 
-const ImageResizeMode = require('ImageResizeMode');
 const ImageStylePropTypes = require('ImageStylePropTypes');
 const NativeMethodsMixin = require('NativeMethodsMixin');
 const NativeModules = require('NativeModules');
@@ -132,8 +131,6 @@ const Image = createReactClass({
   },
 
   statics: {
-    resizeMode: ImageResizeMode,
-
     getSize(
       url: string,
       success: (width: number, height: number) => void,

Libraries/Image/Image.ios.js

@@ -10,7 +10,6 @@
 'use strict';
 
 const ImageProps = require('ImageProps');
-const ImageResizeMode = require('ImageResizeMode');
 const NativeMethodsMixin = require('NativeMethodsMixin');
 const NativeModules = require('NativeModules');
 const React = require('React');
@@ -36,7 +35,6 @@ const Image = createReactClass({
   propTypes: ImageProps,
 
   statics: {
-    resizeMode: ImageResizeMode,
     /**
      * Retrieve the width and height (in pixels) of an image prior to displaying it.
      *

RNTester/js/ImageCapInsetsExample.js

@@ -29,7 +29,7 @@ class ImageCapInsetsExample extends React.Component<{}> {
               height: 60,
             })}
             style={styles.storyBackground}
-            resizeMode={Image.resizeMode.stretch}
+            resizeMode="stretch"
             capInsets={{left: 0, right: 0, bottom: 0, top: 0}}
           />
         </View>
@@ -42,7 +42,7 @@ class ImageCapInsetsExample extends React.Component<{}> {
               height: 60,
             })}
             style={styles.storyBackground}
-            resizeMode={Image.resizeMode.stretch}
+            resizeMode="stretch"
             capInsets={{left: 15, right: 15, bottom: 15, top: 15}}
           />
         </View>

RNTester/js/ImageExample.js

@@ -621,15 +621,15 @@ exports.examples = [
                     <Text style={[styles.resizeModeText]}>Contain</Text>
                     <Image
                       style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.contain}
+                      resizeMode="contain"
                       source={image}
                     />
                   </View>
                   <View style={styles.leftMargin}>
                     <Text style={[styles.resizeModeText]}>Cover</Text>
                     <Image
                       style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.cover}
+                      resizeMode="cover"
                       source={image}
                     />
                   </View>
@@ -639,23 +639,23 @@ exports.examples = [
                     <Text style={[styles.resizeModeText]}>Stretch</Text>
                     <Image
                       style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.stretch}
+                      resizeMode="stretch"
                       source={image}
                     />
                   </View>
                   <View style={styles.leftMargin}>
                     <Text style={[styles.resizeModeText]}>Repeat</Text>
                     <Image
                       style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.repeat}
+                      resizeMode="repeat"
                       source={image}
                     />
                   </View>
                   <View style={styles.leftMargin}>
                     <Text style={[styles.resizeModeText]}>Center</Text>
                     <Image
                       style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.center}
+                      resizeMode="center"
                       source={image}
                     />
                   </View>