Change file permissions for shutdown files

Make the shutdown-checkpoints files readable for platform apps.

Test: Check that shutdown-checkpoint file is created with correct
      permission
    1 Generate a new shutdown-checkpoint file
      This can be done by a reboot from the device power menu
      PS "adb reboot" does not generate a shutdown-checkpoint file
    2 Check the permissions with adb
      $ adb shell ls -laZ /data/system/shutdown-checkpoints | grep shutdown_checkpoints
      drwxr-xr-x  2 system system u:object_r:shutdown_checkpoints_system_data_file:s0  3452 2023-10-19 10:21 .
      -rw-r--r--  1 system system u:object_r:shutdown_checkpoints_system_data_file:s0  1307 2023-10-19 10:21 checkpoints-1697703667771
    3 Check that  the shutdown-checkpoint dir permission is "drwxr-xr-x"
    4 Check that a new shutdown-checkpoint file has been created with
      the permission "-rw-r--r--"
Bug: 306198106
Change-Id: I1639d59f1eb41063c5ac4898738a9f8f5c882028