Flush addPersistentPreferredActivity setting in DPMS

To fix the glitch that "kiosk mode" does not persist if device
is rebooted within 10s after addPersistentPreferredActivity is called.

Test: Manual Test
      1. Using TestDPC to start kisok mode, reboot right away.
      Observed that TestDPC is launched in kiosk mode.
      2. Stop the kiosk mode, reboot without 10s.
         Kisok mode is stopped.

Fix: 28169791

Change-Id: I555fc18efe86380f2e028b698c2bdb01017bf9f5