Fix back navigation in biometric enrollment.

Using the back buttons can cause a crash in at least two cases. Skipping
face enrollment and then starting/stopping any enrollment can lead to
an invalid token and failed HAT request. Backing out of the activity and
restarting it can also lead to using a stale token that fails.

Fix: 179336333
Test: manual on device
Change-Id: I0c1133e4c3d9c97997043ddc9374aa3cfc4f1c97

src/com/android/settings/biometrics/face/FaceEnrollIntroduction.java

@@ -114,9 +114,6 @@ public class FaceEnrollIntroduction extends BiometricEnrollIntroduction {
                 mToken = BiometricUtils.requestGatekeeperHat(this, getIntent(), mUserId, challenge);
                 mSensorId = sensorId;
                 mChallenge = challenge;
-                if (BiometricUtils.isMultiBiometricEnrollmentFlow(this)) {
-                    BiometricUtils.removeGatekeeperPasswordHandle(this, getIntent());
-                }
                 mFooterBarMixin.getPrimaryButton().setEnabled(true);
             });
         }