Don't clear the password field when window focus changes.

This is no longer required since we now clear the password field
when showing the bouncer.  The code used to clear it because
keyguard was relatively static and we didn't want to keep the
password around through an interruption like a phone call or
launching the camera.

Fixes bug 14622312

Change-Id: I4808d51198ba0e6b1e03e26fee22bb08c752e613