Stop allowing nonempty tokens/secrets in "am unlock-user"

It doesn't make sense for the 'am unlock-user' shell command to accept a
token and secret, given that the token is no longer used, and there
isn't really any way for the caller to get the secret if a nonempty
secret is needed.  Therefore, make it so that these two parameters must
be either empty or not specified.  This makes this command more
explicitly only support unlocking users who don't have an LSKF.

Test: atest com.android.server.locksettings
Test: am unlock-user 10
Test: am unlock-user 10 "" ""
Bug: 184723544
Change-Id: Ie4bfac19ba6b753b3434e13be553a654e5a2df35