Support Image resizeMode=repeat on Android

Summary:
`<Image resizeMode="repeat" />` for Android, matching the iOS implementation (#7968). (Non-goal: changing the component's API for finer-grained control / feature parity with CSS - this would be nice in the future)

As requested in e.g. #14158.

Given facebook/fresco#1575, and lacking the context to follow the specific recommendations in facebook/fresco#1575 (comment), I've opted for a minimal change within RN itself.

It's likely that performance can be improved by offloading this work to Fresco in some clever way; but I'm assuming that the present naive approach is still an improvement over a userland implementation with `onLayout` and multiple `<Image>` instances.

- Picking up on a TODO note in the existing code, I implemented `MultiPostprocessor` to allow arbitrary chaining of Fresco-compatible postprocessors inside `ReactImageView`.
- Rather than extensively refactor `ImageResizeMode`, `ReactImageManager` and `ReactImageView`, I mostly preserved the existing API that maps `resizeMode` values to [`ScaleType`](http://frescolib.org/javadoc/reference/com/facebook/drawee/drawable/ScalingUtils.ScaleType.html) instances, and simply added a second mapping, to [`TileMode`](https://developer.android.com/reference/android/graphics/Shader.TileMode.html).
- To match the iOS rendering exactly for oversized images, I found that scaling with a custom `ScaleType` was required - a kind of combination of `CENTER_INSIDE` and `FIT_START` which Fresco doesn't provide - so I implemented that as `ScaleTypeStartInside`. (This is, frankly, questionable as the default behaviour on iOS to begin with - but I am aiming for parity here)
- `resizeMode="repeat"` is therefore unpacked by the view manager to the effect of:
  ```js
     view.setScaleType(ScaleTypeStartInside.INSTANCE);
     view.setTileMode(Shader.TileMode.REPEAT);
   ```
  And the added postprocessing in the view (in case of a non-`CLAMP` tile mode) consists of waiting for layout, allocating a destination bitmap and painting the source bitmap with the requested tile mode and scale type.

Note that as in facebook/react-native#17398 (comment), I have neither updated nor tested the "Flat" UI implementation - everything compiles but I've taken [this comment](facebook/react-native#12770 (comment)) to mean there's no point in trying to wade through it on my own right now; I'm happy to tackle it if given some pointers.

Also, I'm happy to address any code style issues or other feedback; I'm new to this codebase and a very infrequent Android/Java coder.

Tested by enabling the relevant case in RNTester on Android.

| iOS | Android |
|-|-|
| <img src=https://user-images.githubusercontent.com/2246565/34461897-4e12008e-ee2f-11e7-8581-1dc0cc8f2779.png width=300>| <img src=https://user-images.githubusercontent.com/2246565/34461894-40b2c8ec-ee2f-11e7-8a8f-96704f3c8caa.png width=300> |

Docs update: facebook/react-native-website#106

[ANDROID] [FEATURE] [Image] - Implement resizeMode=repeat
Closes facebook/react-native#17404

Reviewed By: achen1

Differential Revision: D7070329

Pulled By: mdvacca

fbshipit-source-id: 6a72fcbdcc7c7c2daf293dc1d8b6728f54ad0249

js/ImageExample.js

@@ -558,18 +558,16 @@ exports.examples = [
                     source={image}
                   />
                 </View>
-                { Platform.OS === 'ios' ?
-                  <View style={styles.leftMargin}>
-                    <Text style={[styles.resizeModeText]}>
-                      Repeat
-                    </Text>
-                    <Image
-                      style={styles.resizeMode}
-                      resizeMode={Image.resizeMode.repeat}
-                      source={image}
-                    />
-                  </View>
-                : null }
+                <View style={styles.leftMargin}>
+                  <Text style={[styles.resizeModeText]}>
+                    Repeat
+                  </Text>
+                  <Image
+                    style={styles.resizeMode}
+                    resizeMode={Image.resizeMode.repeat}
+                    source={image}
+                  />
+                </View>
                 <View style={styles.leftMargin}>
                   <Text style={[styles.resizeModeText]}>
                     Center