[BiometricsV2] Refactor AutoCredentialViewModel

Refactor AutoCredentialViewModelTest and FingerprintEnrollmentViewModel
to kotlin and change LiveData to Flow

Bug: 286197659
Test: atest -m CredentialModelTest
Test: atest -m AutoCredentialViewModelTest
Test: atest -m FingerprintEnrollmentViewModelTest
Test: atest -m FingerprintEnrollmentActivityTest
Test: atest -m biometrics-enrollment-test
Change-Id: I84bab0b46e023303c0046a6ae6886ab1cf9458b8