Put development flag behind IS_ENG check

Per discussion, this flag was only ever needed for development
debugging.  Therefore for optimization (not having lots of apps register
an observer to DeviceConfig) and potentially reducing problems caused by
having multiple packages all reading from one namespace, put behind an
ENG build check

Test: manual
Bug: 265145724
Change-Id: Ia6f2ee781c431088cbb3417c1b6bac86cfce4c13