Clear mGuestRestrictions before loading from disk

Only guest user restrictions that are set to true are saved in
writeUserListLP. When loading <guestRestrictions>, they are applied
on top of mGuestRestrictions filled with default values.
Restrictions set to false are reset and this was causing the issue.

Bug: 33250860
Test: Manual
Change-Id: I6e5c792a3adcf2116bb26fd5feba551d7edcb28e