ImagePicker: Handle maxfiles correctly.

Adds an option composeMenu.js to pass in a maxFiles number.

This is used to make sure the user can only select the correct
number of files subject to our max of 4.

src/compose/ComposeBox.ios.js

@@ -407,6 +407,7 @@ class ComposeBox extends PureComponent<Props, State> {
               onImageSelect={this.handleImageSelect}
               disableCamera={draftImages && Object.keys(draftImages).length >= 4}
               disableUpload={draftImages && Object.keys(draftImages).length >= 4}
+              maxFiles={4 - Object.keys(draftImages).length}
             />
           </View>
           <View style={styles.composeText}>

src/compose/ComposeMenu.js

@@ -20,6 +20,7 @@ type Props = {
   onImageSelect: Object => void,
   disableUpload?: boolean,
   disableCamera?: boolean,
+  maxFiles?: number,
 };
 
 /**
@@ -76,13 +77,14 @@ class ComposeMenu extends PureComponent<Props> {
 
   handleImageUpload = () => {
     const { handleImagePickerResponse } = this;
+    const { maxFiles } = this.props;
     ImagePicker.openPicker({
       mediaType: 'photo',
       multiple: true,
       compressImageMaxWidth: 1600,
       compressImageMaxHeight: 1600,
       compressImageQuality: 0.7,
-      maxFiles: 4,
+      maxFiles,
     })
       .then(images => {
         handleImagePickerResponse({