fix(mobile): handle image stream completion when no image is emitted (#25984)

* Fix image cancellation to be stream-scoped instead of widget-scoped

* fix(OneFramePlaceholderImageStreamCompleter): make onLastListenerRemoved callback synchronous with removing the last listener

* fix(OneFrameMultiImageStreamCompleter): remove unnecessary blank line in code

* fix(OneFramePlaceholderImageStreamCompleter): cancel pending requests when only cache listener remains

* fix(OneFrameMultiImageStreamCompleter): ensure onLastListenerRemoved callback is invoked only once

mobile/lib/presentation/widgets/images/remote_image_provider.dart

@@ -32,7 +32,7 @@ class RemoteImageProvider extends CancellableImageProvider<RemoteImageProvider>
         DiagnosticsProperty<ImageProvider>('Image provider', this),
         DiagnosticsProperty<String>('URL', key.url),
       ],
-      onDispose: cancel,
+      onLastListenerRemoved: cancel,
     );
   }
 
@@ -76,7 +76,7 @@ class RemoteFullImageProvider extends CancellableImageProvider<RemoteFullImagePr
         DiagnosticsProperty<ImageProvider>('Image provider', this),
         DiagnosticsProperty<String>('Asset Id', key.assetId),
       ],
-      onDispose: cancel,
+      onLastListenerRemoved: cancel,
     );
   }