android: Enable receiving 'shares' from other apps.

All concerning bugs preventing the launch of this issue have been
fixed in the previous commits.

This does not include Sharing Multiple Files.

Fixes: zulip#117

android/app/src/main/AndroidManifest.xml

@@ -66,13 +66,11 @@
             android:label="@string/app_name"
             android:launchMode="standard"
         >
-            <!-- Disabled while the feature is experimental.  See #117 and #4124.
             <intent-filter>
                 <action android:name="android.intent.action.SEND" />
                 <category android:name="android.intent.category.DEFAULT" />
                 <data android:mimeType="*/*" />
             </intent-filter>
-            -->
         </activity>
 
         <service android:name=".notifications.NotificationIntentService" />