Add ALERT SoundType enum value (flutter#20139)

flutter#62143

Author: tvolkert

shell/platform/android/io/flutter/embedding/engine/systemchannels/PlatformChannel.java

@@ -430,7 +430,8 @@ public interface PlatformMessageHandler {
 
   /** Types of sounds the Android OS can play on behalf of an application. */
   public enum SoundType {
-    CLICK("SystemSoundType.click");
+    CLICK("SystemSoundType.click"),
+    ALERT("SystemSoundType.alert");
 
     @NonNull
     static SoundType fromValue(@NonNull String encodedName) throws NoSuchFieldException {