Fix DO user restrictions persisting in UMS

<device_policy_restrictions> was *not* written in <guestRestrictions>.

<guestRestrictions>
    <restrictions no_sms="true" />
</guestRestrictions>
<device_policy_restrictions no_fun="true" no_adjust_volume="true" />

Fix the loading code.

Test: Manually set DO/PO restrictions, rebooted the device and checked with
dumpsys.

Bug 30949168

Change-Id: I8708a0b716852e3d981d2fe866f6628c0df80dcb