2/n: Add default implementation for multi-biometric enroll

1) Adds a layout for multi-biometric selection in BiometricEnrollActivity
2) Adds widgets for checkboxes
3) Shows ConfirmLock*/ChooseLock* for multi-biometric devices in
   BiometricEnrollActivity
4) finish()'s when loses foreground
5) Adds default string for ChooseLock* and multi-biometrics, e.g.
   "Set up Password + Biometrics", as well as associated plumbing
   to bring the user back to BiometricEnrollActivity once the
   credential is enrolled
6) When max templates enrolled, checkbox becomes disabled and
   description string is updated

Bug: 162341940
Bug: 152242790
Fixes: 161742393

No effect on existing devices with the following:
Test: adb shell am start -a android.settings.BIOMETRIC_ENROLL
Test: SUW
Test: make -j RunSettingsRoboTests

Exempt-From-Owner-Approval: Biometric-related change
to EncryptionInterstitial

Change-Id: I855460d50228ace24d4ec5fbe330f02ab406cc02