[iOS] Fixes press key message leaks

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;