Fixed biometric enable for keyguard status not correctly updated

The conflatedCallbackFlow may drop data and only send the latest update.

Use callbackFlow instead to fix it.

Bug: 392289077
Test: atest BiometricSettingsRepositoryTest.kt
Flag: com.android.settings.flags.biometrics_onboarding_education
Change-Id: I76d535a858fcf9af1bd544763a846d6df76d28b5