Attempt to unlock users with null token.

When starting a locked user, try unlocking their storace will a null
token, which will typically succeed if there is an insecure
lockscreen (no PIN or pattern).

For users with a secure lockscreen, pass through a stub token for
now to indicate that it came from a user challenge.  Eventually we'll
hook that up to gatekeeperd.

Without this, we were only unlocking users with secure lockscreens.

Bug: 25943941
Change-Id: Ia0324d50f43f55dfe0b8366793ddc5d25d885922