Allow persistent changes to the vendor overlay theme

This allows the overlay being used to be changed without a new build
but still will require a reboot to take effect. Should no longer be
needed once hierarchical resources are in place, and can be removed.

Also fix check in fd_utils to point at correct location.

Test: Manual
Bug: 32721178
Change-Id: I2a63aea0c87791c8eb845d735cb1182716c8174d