sharing: Stop passing width and height props to Image component.

As of RN v0.62.2, which we're on currently, we've technically been
allowed to choose between setting `width` and `height` as their own
props, or putting them in the `style` prop. Here's the relevant bit
of the type definition:

```js
  /**
   * See https://facebook.github.io/react-native/docs/image.html#style
   */
  style?: ?ImageStyleProp,

  // Can be set via props or style, for now
  height?: ?DimensionValue,
  width?: ?DimensionValue,
```

But the `width` and `height` props aren't in the documentation [1];
it seems that the most proper place for them is in the `style` prop.
And when we upgrade to RN v0.63, which we hope to do soon, we'll no
longer be allowed to choose; using the `style` prop will be the only
option.

We've already been passing `width` and `height` in the `style` prop,
so just fall back to doing that by itself.

[1] https://reactnative.dev/docs/0.62/image

src/sharing/ShareToPm.js

@@ -162,12 +162,7 @@ class ShareToPm extends React.Component<Props, State> {
     let sharePreview = null;
     if (sharedData.type === 'image') {
       sharePreview = (
-        <Image
-          source={{ uri: sharedData.sharedImageUrl }}
-          width={200}
-          height={200}
-          style={styles.imagePreview}
-        />
+        <Image source={{ uri: sharedData.sharedImageUrl }} style={styles.imagePreview} />
       );
     }
 

src/sharing/ShareToStream.js

@@ -163,12 +163,7 @@ class ShareToStream extends React.Component<Props, State> {
         <ScrollView style={styles.wrapper} keyboardShouldPersistTaps="always">
           <View style={styles.container}>
             {sharedData.type === 'image' && (
-              <Image
-                source={{ uri: sharedData.sharedImageUrl }}
-                width={200}
-                height={200}
-                style={styles.imagePreview}
-              />
+              <Image source={{ uri: sharedData.sharedImageUrl }} style={styles.imagePreview} />
             )}
             <AnimatedScaleComponent visible={isStreamFocused}>
               <StreamAutocomplete filter={stream} onAutocomplete={this.handleStreamAutoComplete} />