Fix system process crash

verifyKnownUsers() might fail due to database error
in low storage which will crash the system process
and cause boot loop.

Bug: 300568641
Test: 1. put large files under /data/local/tmp
         to cause low storage
      2. device can reboot successfully
Change-Id: If1a3ac06b2c02b273afa47e6b0a90bf08408fb6b