Allow ueventd to set verity.* properties

On dm-verity errors, we catch uevents in ueventd and set the value
for a matching verity.* property. Allow ueventd to actually change
property values.

Needed by changes from
  Ibb82953594d234f81ad21c40f524190b88e4ac8f

Change-Id: I79bc90733edf8a45b27e64795f4adfbb3bc028dc