Prevent lock icon a11y when unlocking.

When we are unlocking the lock icon is reading out a content
description, indicating that it is locked. In consequence, talkback will
often read "device locked. device unlocked" when unlocking the phone
when talk back is on. A lot of the keyguard states don't seem viable to
use here, but I found that the bouncer indicates that it is animating
away here.

Fixes: 284902478
Test: with talk back on, open notification in LS and authenticate
Test: with talk back on, swipe up bouncer and authenticate
Change-Id: I8ae54095c55a5ca0816878f27b368c3a9abd73e3