Fix FingerprintEnrollIntroFragment buttons

1. Fix FingerprintEnrollIntroFragment button status not sync after
   rotation.
2. Finetune code orders in FingerprintEnrollmentActivity.onCreate(),
3. Refine some API names and java doc

Bug: 260205364
Test: atest AutoCredentialViewModelTest
            FingerprintEnrollIntroViewModelTest
Test: Manually test rotation with different display size
Change-Id: Ib992e10a1c90e3bbac6cf73114df4e96b5ab0a03