[SPARK-3410] The priority of shutdownhook for ApplicationMaster should not be integer literal

I think, it need to keep the priority of shutdown hook for ApplicationMaster than the priority of shutdown hook for o.a.h.FileSystem depending on changing the priority for FileSystem.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2283 from sarutak/SPARK-3410 and squashes the following commits:

1d44fef [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-3410
bd6cc53 [Kousuke Saruta] Modified style
ee6f1aa [Kousuke Saruta] Added constant "SHUTDOWN_HOOK_PRIORITY" to ApplicationMaster
54eb68f [Kousuke Saruta] Changed Shutdown hook priority to 20
2f0aee3 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-3410
4c5cb93 [Kousuke Saruta] Modified the priority for AM's shutdown hook
217d1a4 [Kousuke Saruta] Removed unused import statements
717aba2 [Kousuke Saruta] Modified ApplicationMaster to make to keep the priority of shutdown hook for ApplicationMaster higher than the priority of shutdown hook for HDFS