Bug 16152902: Disabling FUL in keyguard and screen lock settings

This is the framework change needed to switch from Face Unlock as
a standard unlock mechanism to Face Unlock as a trustlet.  This
forces isBiometricWeakInstalled() to return false such that neither
keyguard nor Screen Lock Settings thinks it exists.  This means FUL
is no longer a screen lock option.

If Face Unlock was already enabled and someone updates to this build,
the screen lock automatically switches to the backup lock.  That
functionality was already built-in so everything would continue to
work if Face Unlock decided to disappear from someone's device.

Change-Id: I407f6e1d75bfe40c7142c2508c2d68867e02d858