[XR] Add a default camera usage description

ProjectSettings/ProjectSettings.asset

   enableInternalProfiler: 0
   logObjCUncaughtExceptions: 1
   enableCrashReportAPI: 0
-  cameraUsageDescription: 
+  cameraUsageDescription: Camera required for AR
   locationUsageDescription: 
   microphoneUsageDescription: 
   switchNetLibKey: