Include default icons in sample app

Author: Adam Schlesinger

com.onesignal.unity.android/Editor/OneSignalConfig.plugin/res/drawable-xxhdpi-v11.meta renamed to OneSignalExample/Assets/Plugins/Android/OneSignalConfig.meta

@@ -0,0 +1,6 @@
+Android Setup
+=====================
+
+1. Replace the example small notification icons in the res folder with your own with by follwing the insturctions below.
+  - https://documentation.onesignal.com/docs/android-notification-customizations#small-notification-icon
+2. The large notification icon is optional, remove or replace res/drawable-xxhdpi-v11/ic_onesignal_large_icon_default.png.