[iOS] Fixes press key message leaks (#29354)

Author: zhongwuzw

shell/platform/darwin/ios/framework/Source/FlutterChannelKeyResponder.mm

@@ -132,7 +132,7 @@ - (void)handlePress:(nonnull FlutterUIPressProxy*)press
   NSString* characters = getEventCharacters(press.key.characters, press.key.keyCode);
   NSString* charactersIgnoringModifiers =
       getEventCharacters(press.key.charactersIgnoringModifiers, press.key.keyCode);
-  NSMutableDictionary* keyMessage = [@{
+  NSMutableDictionary* keyMessage = [[@{
     @"keymap" : @"ios",
     @"type" : type,
     @"keyCode" : @(press.key.keyCode),
@@ -141,7 +141,7 @@ - (void)handlePress:(nonnull FlutterUIPressProxy*)press
     @"charactersIgnoringModifiers" : charactersIgnoringModifiers == nil
         ? @""
         : charactersIgnoringModifiers,
-  } mutableCopy];
+  } mutableCopy] autorelease];
   [self.channel sendMessage:keyMessage
                       reply:^(id reply) {
                         bool handled = reply ? [[reply valueForKey:@"handled"] boolValue] : true;