The old wpa_supplicant.conf format used a special URI prefix "keystore://" for the private_key value to indicate when to load things from keystore. The new format uses an OpenSSL ENGINE to perform operations with the private key, so we don't need the special URI prefix. This changes enables that usage and also supports migrating the old style configuration to the new style. Change-Id: Ibdf2322743eaa129bd2aa5e874f197b573714b57