Persist Standby Buckets after System defaults set

Also repair defaults if persisted file is missing on boot.

Fixes: 123206562
Test: manual ("adb shell reboot" within 20 min of a fresh wipe
              "adb shell dumpsys usagestats"
	      System apps bucket levels equal 10)

Change-Id: I0403099c899d181ef6206cc9f5516c25213edc9c