Remove entropy from service_contexts.

Commit: 9287e0dd272b85b475e33bcbd7d868517a0f98f9 removed the registration
of EntropyMixer with servicemanager, so it no longer needs a context.

Bug: 18106000

Cherry-pick of commit: 7cfef98c

Change-Id: I9aeb35e7ffde75090f4234ea193514fb883b1425