Fix several challenge paths

1) Combined settings page should finish() when losing foreground
   (but not if fingerprint or face settings are launched from it)
2) Challenge should be generated when entering FaceSettings or
   FingerprintSettings each time. This allows us to leave the existing
   revokeChallenge paths in FaceSettings and FingerprintSettings.

Fixes: 186257202
Test: Manual

Change-Id: I1b6452f787279001a71628f220c01fcb86b88f3d