Look up user-specific device policy for disabling camera

Camera can be disabled on a per-user basis by device admins.
Changed the system property format to be per-user so that
the policy can be applied based on calling user.

TODO: Ideally this policy information should be pulled from the
DevicePolicyManager rather than relying on system properties.
Property changes will not be applied immediately either, if
there's no listener.

Bug: 19345698
Change-Id: Ia00034726260bc9ff28ac592f20a27b5c9a77d58