Further Credentials-related clean-up

Additional clean-up work related to removal of screenlock dependency
from the credentials installation flow:
* Move the CredentialStorage class to security/ so that Enterprise team
  owners could review changes to it.
* Remove the ConfigureKeyGuardDialog class as it is no longer used.
* Remove attempt to unlock KeyStore from VPN settings.
* Remove intents that will no longer be sent from the manifest.

Bug: 120901345
Test: m -j RunSettingsRoboTests
Test: Manual with CtsVerifier
Change-Id: Ia708ede3366892d74c148f3712a63858d5ab53b7