[Fingerprint] Show skip button during SUW

Show a skip button during enrolling in case the user entered the flow
accidentally. The skip button will send the result code RESULT_SKIP,
which is equal to RESULT_FIRST_USER + 1, back to the starting
activity.

Bug: 22666389
Change-Id: I230b04e5150214c31536ac282d56b7b490c85ac1