Fix inoperative "Forgot pattern" button when face unlock enabled

This fixes a bug where the forgot pattern button wasn't working because
the logic for face unlock was interfering with determining the proper
backup to use.

The fix:
- adds a new state to SecurityMode so we have an initial condition
we can check for.
- passes the current mode to SecurityModel.getBackupSecurityMode() so
it relies on the current state.
- prevents face unlock from invoking callbacks that change state
once we're no longer showing face unlock.

Fixes bug 7346989

Change-Id: I4e64515efbbad712f11c820e690b458f352bf46c