android: Enable receiving 'shares' from other apps.

This feature was marked as experimental because of several small
bugs. These have been fixed in the preceeding commits. So, remove
the comment that disables it.

Fixes: #117

android/app/src/main/AndroidManifest.xml

@@ -33,13 +33,11 @@
             <category android:name="android.intent.category.BROWSABLE" />
             <data android:scheme="zulip" android:host="login" />
         </intent-filter>
-        <!-- 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" />