Pass Config pointer into AuthManager

AuthManager needs to have logic for re-claming devices. This requires
persistent storage for secret and owner. It's going to be easier to
handle logic there than outside.

BUG=25766815

Change-Id: Icc417f23715b48461098503fd241cee534d2225c
Reviewed-on: https://weave-review.googlesource.com/1949
Reviewed-by: Vitaly Buka <vitalybuka@google.com>