Changes to biometric sensor interface in lockscreen

This cleans up the biometric sensor interface - the interface between
lockscreen and Face Unlock.  Not only does it document the interface,
but it also makes two noteworthy changes to the interface:

1) Instead of calling mBiometricUnlock.start() with a parameter to
tell it whether to suppress itself, lockscreen makes all of the
decisions about whether the biometric unlock should be started or not
and only calls start() if it should be started.  Passing a parmeter to
tell a function to not start itself was strange, but it was a
necessary intermediate step in the process of fixing this interface.

2) Instead of calling mBiometricUnlock.initializeView() with a top
view that the biometric unlock should attach to, lockscreen now
provides the biometric unlock with the actual view it is allowed to
work in.  This keeps lockscreen in control of where the biometric
sensor is allowed to display.

A few things were also cleaned up within the Face Unlock
implementation of the biometric interface:

1) Changes needed to match the requirements of the improved biometric
sensor interface, including moving the functions into an order that
makes more sense.

2) The bind() function was only being called from start(), which has
turned into only a couple of lines of code, so the bind() code has
been just put inline into the start() function, which mirrors the
stop() function which has the unbind() code in it.

3) The showArea() function was really just one line of code with a
check.  It was being called from two places.  The showArea() code is
now just written inline in those two places, which makes the code
much easier to follow.

4) Renamed occurrences of FaceLock to Face Unlock.

Change-Id: Ie95ce21dcc77170381af9ce320f2675dfe249741