After YARN-4170, Application Priority is available via heartbeat call. Update this information in AM sothat client can fetch this information via JobStatus (JobReport) call.
This is as per the discussion happened in MAPREDUCE-5870.
AM need to be notified with priority in AllocateResponse
RMContainerAllocator#handleJobPriorityChange expects application_priority always
Support for passing Job priority through Application Submission Context in Mapreduce Side