Accessibility change when Trusted Face icon shows.

When unlocking device, if Trusted Face is on, "Scanning face" should be
automatically announced.
Also change the contentDescription for face icon, then when clicking the face icon,
instead of "unlock button", "Scanning face" will be talked back.

Test: manual
Bug:74328543
Change-Id: I94f9faa45bf5f6f7ceaf7df5acab0a970f651395