Merge changes from topic "b409835922" into main

* changes:
  init: reboot: Kick off RebootMonitorThread() upon sys.shutdown.requested
  init: reboot: Create a function for starting RebootMonitorThread()
  init: reboot: Remove `reboot_target` arg from RebootMonitorThread()
  init: reboot: Improve RebootMonitorThread() evasive actions
  init: reboot: Simplify the RebootMonitorThread()
  init: reboot: Max ro.build.shutdown_timeout at 10s as the "clean" timeout