Adding accessibility support to the PIN lock screen.

1. The password lock screen is accessible and with this
   change the PIN lock screen is accessibile as well.
   This is enough to cover the enterprise use case of
   imposed lock of the deivce. we will hide the options
   for pattern since it is hard for use by a blind person.
   We may reconsider this for subsequent releases.

bug:4978246

Change-Id: I67ef783b799ffd64ebff6cdb614c03025fc911e6