Add POST_NOTIFICATIONS permission to AndroidManifest of RNTester

Summary:
In D38850138 (63a4539) I added a new example of the runtime permission `POST_NOTIFICATIONS` to RNTester, but forgot to add this permission to AndroidManifest.xml.

Changelog:
[Android][Added] - Add POST_NOTIFICATIONS permission to AndroidManifest of RNTester

Reviewed By: fkgozali

Differential Revision: D41204931

fbshipit-source-id: 80e5d526406715fd7570cc94cbf384858ff05aed

packages/rn-tester/android/app/src/main/AndroidManifest.xml

@@ -18,6 +18,7 @@
   <!--Just to show permissions example-->
   <uses-permission android:name="android.permission.CAMERA"/>
   <uses-permission android:name="android.permission.READ_CALENDAR"/>
+  <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
 
   <!--
   android:icon is used to display launcher icon on mobile devices.