Show "Done" button to prevent user from getting stuck

When the user has already reached max enrollments, we should show a done
button. It's possible to get to the introduction page (back button during
SUW, or launching android.settings.BIOMETRIC_ENROLL).

Bug: 111548033
Test: Done button is shown and works as expected when max enrollments
Change-Id: Ia70c21f63606393116e222ca940ee93bcfb25995