Move multi-biometric logic to base fp intro activity

Ensures that the relevant multi-biometric logic--specifically, logic to
ensure that fingerprint enroll is shown exactly once after face
enroll--is present in both the SUW and non-SUW versions of the
fingerprint enroll introduction activity. This fixes an issue where
fingerprint enroll would occur multiple times when triggering the
multi-biometric enroll flow outside of SUW (e.g. via an enroll intent
action).

Test: CTS-Verifier: Biometric Tests > 2a: Strong Biometrics + Crypto

Bug: 193601823
Change-Id: I17c09e9a72ced2f23b0b279ea51a6aa209668485
(cherry picked from commit 0a8916a8)