Update Wifi to use new keystore function

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