Use allow_private_profile flag to guard private space API changes

Currently, the BiometricPrompt API setAllowBackgroundAuthentication is
guarded by the enableBiometricsToUnlockPrivateSpace feature flag. This
does not follow the pattern used by other private space API chagnes
wherein all the APIs are guarded by the allowPrivateProfile flag and the
implementation is guraded by both the allowPrivateProfile and the
individual feature flag. With this change, we move the
setAllowBackgroundAuthentication to use the allowPrivateProfile flag to
follow the same model used by other private sapce changes as mentioned
above.

Bug: 312184187
Test: atest CredentialInteractorImplTest
Change-Id: I8ce46920003956624809f201ba151fad16f3565b