Changed ClientAdminEndpoints.setSecurityContextAccessor method access modifier to public

It should be public (like in the [ApprovalsAdminEndpoints](https://github.com/cloudfoundry/uaa/blob/6d214c2728e076523419e3fb0c20c8395485cb03/common/src/main/java/org/cloudfoundry/identity/uaa/oauth/approval/ApprovalsAdminEndpoints.java) or [UaaAuthorizationRequestManager](https://github.com/cloudfoundry/uaa/blob/6d214c2728e076523419e3fb0c20c8395485cb03/common/src/main/java/org/cloudfoundry/identity/uaa/oauth/UaaAuthorizationRequestManager.java)) to allow a custom [SecurityContextAccessor](https://github.com/cloudfoundry/uaa/blob/6d214c2728e076523419e3fb0c20c8395485cb03/common/src/main/java/org/cloudfoundry/identity/uaa/security/SecurityContextAccessor.java) to be used.