Fixed test failed on aosp

There is no mock for
FaceFeatureProvider.getEnrollActivityClassProvider() in aosp which
caused the null exception test failure.

Use FaceEnroll class name to lauch enroll activity directly to fix this
issue.

Bug: 388933801
Test: atest FaceSettingsEnrollButtonPreferenceControlleTest
Flag: com.android.settings.flags.biometrics_onboarding_education
Change-Id: Ibbbbe888f95c1ece0372ac31435ce6817cba45b5