Set OSInAppMessagePrompt as public

This should be public as OSInAppMessageAction has a public getPrompts
method which returns list of the type OSInAppMessagePrompt.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessagePrompt.java

@@ -1,6 +1,6 @@
 package com.onesignal;
 
-abstract class OSInAppMessagePrompt {
+public abstract class OSInAppMessagePrompt {
 
     private boolean prompted = false;