CDD: Clarify secure lock screen requirements.

As some device implementations started to add or modify the
authentication methods for the lock screen, and more APIs
are making an assumption on the security of the lock screen
credentials, we are clarifying the requirements of what
is a secure lock screen.

Bug: 27246863

Change-Id: I618999405a862125348758ae34a40701bfaa1b62
4 files changed