Add protected BiometricPrompt API to allow default title

ConfirmDeviceCredentials sometimes shows up without a title. Since CC
is now using BiometricPrompt, we need at least a private API that allows
the client to have the dialog show a default title.

Bug: 111461540

Test: Manual test with modified BiometricPromptDemo, to launch CC with
      empty string

Change-Id: I02a3c9327635c04f201f76754f7c0e1135e5ff36