Clean up ex-users in lock settings db

Just in case a userid was not properly cleaned
when the user was removed, make sure it is
cleaned up when a new user takes up the same
userid. This prevents inconsistent lockscreen
state and avoids a crash in Settings when trying
to set a password for the new user.

Fixes: 29412112
Change-Id: Ic4f0efbb97786b0290c74325b28c9d74825e9e53