fix(storage, android): fix an issue that could crash the app when con…

…current calls to removeEventListeners were happening (#16996)

packages/firebase_storage/firebase_storage/android/src/main/java/io/flutter/plugins/firebase/storage/FlutterFirebaseStoragePlugin.java

@@ -156,16 +156,26 @@ private String registerEventChannel(String prefix, String identifier, StreamHand
     return identifier;
   }
 
-  private void removeEventListeners() {
-    for (String identifier : eventChannels.keySet()) {
-      eventChannels.get(identifier).setStreamHandler(null);
+  private synchronized void removeEventListeners() {
+    // Create a list to hold the keys to remove after iteration
+    List<String> eventChannelKeys = new ArrayList<>(eventChannels.keySet());
+    for (String identifier : eventChannelKeys) {
+      EventChannel eventChannel = eventChannels.get(identifier);
+      if (eventChannel != null) {
+        eventChannel.setStreamHandler(null);
+      }
+      eventChannels.remove(identifier);
     }
-    eventChannels.clear();
 
-    for (String identifier : streamHandlers.keySet()) {
-      streamHandlers.get(identifier).onCancel(null);
+    // Create a list to hold the keys to remove after iteration
+    List<String> streamHandlerKeys = new ArrayList<>(streamHandlers.keySet());
+    for (String identifier : streamHandlerKeys) {
+      StreamHandler streamHandler = streamHandlers.get(identifier);
+      if (streamHandler != null) {
+        streamHandler.onCancel(null);
+      }
+      streamHandlers.remove(identifier);
     }
-    streamHandlers.clear();
   }
 
   private FirebaseStorage getStorageFromPigeon(