[iOS] Fixes leaks of presses key message (flutter#21987)

Related PR: flutter#20972

Author: zhongwuzw

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

@@ -1031,14 +1031,14 @@ - (void)dispatchPresses:(NSSet<UIPress*>*)presses API_AVAILABLE(ios(13.4)) {
           press.phase == UIPressPhaseChanged) {
         continue;
       }
-      NSMutableDictionary* keyMessage = [@{
+      NSMutableDictionary* keyMessage = [[@{
         @"keymap" : @"ios",
         @"type" : @"unknown",
         @"keyCode" : @(press.key.keyCode),
         @"modifiers" : @(press.key.modifierFlags),
         @"characters" : press.key.characters,
         @"charactersIgnoringModifiers" : press.key.charactersIgnoringModifiers
-      } mutableCopy];
+      } mutableCopy] autorelease];
 
       if (press.phase == UIPressPhaseBegan) {
         keyMessage[@"type"] = @"keydown";