Persist whether the job has started in a system property.

In this way, this flag survives across system server restarts.
Therefore, `cleanup` doesn't clean up staged files when the system
server dies and is restarted, if the job has ever started.

Bug: 311377497
Test: -
  1. Install an ART apex.
  2. adb shell cmd jobscheduler run android 27873781
  3. Wait for the job to finish.
  4. adb shell stop && adb shell start
  5. adb shell pm art cleanup
  6. Check that the staged files are still there.
Change-Id: I451122845341ba52f0574052a37ad95b0a9ef9c4