notif [nfc]: Add Android-only asserts in NotificationDisplayManager methods

NotificationDisplayManager's public methods only get invoked on
Android, and their implementations basically assume the platform is
Android. So, add these asserts, to make that assumption clearer and
to fail early if we accidentally call one of these on iOS.

When we eventually take more hands-on control of notifications on
iOS, we might either adapt this class to support iOS too, or make a
new class for iOS and make this one's name and interface clearer
that it's Android-only.

Discussion:
  https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/NotificationDisplayManager.20Android-only/near/2099196

Author: tomlin7

lib/notifications/display.dart

@@ -217,10 +217,12 @@ class NotificationChannelManager {
 /// Service for managing the notifications shown to the user.
 class NotificationDisplayManager {
   static Future<void> init() async {
+    assert(defaultTargetPlatform == TargetPlatform.android);
     await NotificationChannelManager.ensureChannel();
   }
 
   static void onFcmMessage(FcmMessage data, Map<String, dynamic> dataJson) {
+    assert(defaultTargetPlatform == TargetPlatform.android);
     switch (data) {
       case MessageFcmMessage(): _onMessageFcmMessage(data, dataJson);
       case RemoveFcmMessage(): _onRemoveFcmMessage(data);
@@ -490,6 +492,8 @@ class NotificationDisplayManager {
     required BuildContext context,
     required Uri url,
   }) {
+    assert(defaultTargetPlatform == TargetPlatform.android);
+
     final globalStore = GlobalStoreWidget.of(context);
 
     assert(debugLog('got notif: url: $url'));
@@ -518,6 +522,7 @@ class NotificationDisplayManager {
   /// generated with [NotificationOpenPayload.buildUrl] while creating
   /// the notification.
   static Future<void> navigateForNotification(Uri url) async {
+    assert(defaultTargetPlatform == TargetPlatform.android);
     assert(debugLog('opened notif: url: $url'));
 
     NavigatorState navigator = await ZulipApp.navigator;