BiometricEnrollIntro should use non-blocking generateChallenge

GenerateChallenge used to block when showing the credential screen.
Now that GenerateChallenge is moved to after the credential screen
is shown, we need to delay the next button instead. This is generally
non percievable to the user, but this is more robust against busy
system server.

Fixes: 161325267
Test: Enroll fingerprint/face device
Change-Id: I0fbbef8bf469e32bed251acf22556ad2ea8e2933