Save per-user camera policy in different system properties

If a device admin for a user disables the device cameras,
only apply that policy to that user and not globally.

Corresponding change in CameraService looks into the
per-user system property.

This also fixes the bug that managed profile owner is
able to disable camera for the personal profile.

Bug: 19345698
Change-Id: Ibd5e438544a0409f26087ced247d50c706fcf843