Update a11y label for unlock icon on lock screen.

Fixes: 226966905
Test: manually, enroll face auth without bypass, enable talk back, lock the phone by pressing the power button, press power button to open locked lockscreen, unlock using face, verify the label being read out for unlock icon is "Unlocked" instead of "Unlock"
Change-Id: I30bbd5c68ad6bdf0a00c4c72c40e55f71d8f729f