poweroffalarm_app: Remove levelFrom attribute

levelFrom is used to determine the level (sensitivity + categories)
for MLS/MCS. If set to all, level is determined from both UID and
user ID. This is bad for poweroffalarm, as it needs to be able to
write to /persist/alarm/data which has a context without mls_level:
  u:object_r:persist_alarm_file:s0
instead of
  u:object_r:persist_alarm_file:s0:c0,c256,c512,c768

Change-Id: I9a8b706cdedc090281e4b5542eb34816b7ff338e