Enabling Face Unlock for user switching (bug 8495282)

We had been intentionally disabling FUL when switching to a new user.
The reason was that we were only getting a signal when the user switch
started, not when it completed.  If we started FUL at the beginning of
the user switch, it could be completed by the time the user switch
completed.  We now have a signal to tell us that the switch completed.

Prior to this change, FUL would start whenever keyguard is created.
For a user switch, keyguard is recreated when we get
onUserSwitching(), but we don't want FUL to start until we get
onUserSwitchComplete().  So with this change, if onResume() happens
because of a user switch it doesn't start FUL until we get
onUserSwitchComplete().

Change-Id: I1d3da0a32b9b4cf0cfa5c577f1697d2a41ee4fda