android: Trust user-supplied CAs, take 2

The previous attempt at this (cffb112, #474) set the configuration
used by the HTTP implementation from the Android SDK, and by some
third-party HTTP implementations.  But in the bulk of the app's code,
we use the HTTP implementation from the Dart standard library, and
by default that does not consult the Android configuration.

Happily, Flutter has an opt-in feature to apply the same configuration
to the Dart standard library:
  https://chat.zulip.org/#narrow/stream/48-mobile/topic/flutter.3A.20user-added.20certs/near/1716845

So use that.

Fixes: #461

android/app/src/main/AndroidManifest.xml

@@ -38,6 +38,9 @@
                 <data android:scheme="zulip" android:host="login" />
             </intent-filter>
         </activity>
+        <meta-data
+            android:name="io.flutter.network-policy"
+            android:resource="@xml/network_security_config" />
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data