Face Unlock is shown correctly during a phone call - fix b/7271718

If the user was in the middle of a phone call and went to the lock screen
it would show a black box with an X, but Face Unlock wouldn't pop up and
the X was unresponsive.

There were a few issues causing this.  The X on the default view wasn't a
button, so it has been changed to a button which will go to the backup
lock.  The concept of show() and hide() in FaceUnlock.java are obsolete
because Face Unlock is no longer being overlayed on top of the backup so
there's isn't a black box to show or hide. In addition, since it's not being
overlayed, Face Unlock doesn't cover the backup lock so fading to the backup
looks janky.  The flip animation is more appropriate.

Change-Id: I730aa4bbce42b4656ee1bce61352b8aefbd6892d