Update Settings together with generateChallenge/revokeChallenge

The internal implementation of generate/revoke in system_server is now
asynchronous. To keep existing clients working, the manager classes
introduce a blocking version of the generateChallenge calls. This change
updates Settings to use the backward-compatible blocking calls.

Bug: 157790417

Test: Enroll fingerprint/face
Test: After enrollment, toggle setFeature or do subsequent enrollment
      in face/fingerprint settings
Change-Id: Ib4dfdc5f12530b938ab9b1745f5a19cd9e2eceee