Do not apply the PRNG workarounds if an explicit seed is given

If the caller seeds the engine, do NOT override that seed with another
from urandom.

This is a companion change to Ib6bf4478fc1ae3d16eefa4eb2ad90f1f3e9de021,
and should be reverted along with it when the proper fix appears.

Change-Id: I0a9ba35d74af89c91b6aafa35cc94001372794e1