Update fingerprint enrolling flow

1. Make FingerprintEnrollFinish stay on the back stack during "add
   another"
2. Remove skip button from the enrolling screen, since the user can
   now back out of the enrollment flow, or back to
   FingerprintEnrollFinish and press "done" there.

Test: Run FingerprintEnrollFinishTest with `am instrument`
Bug: 32054268
Change-Id: Icaf017952d6582a14b4d0904b7ee67ad22e5280d