Talkback focusing on lock screen should have a label

Talkback or similar tools focusing on lock screen is currently
"Unlabelled". Now that the user can select the whole lock screen in
order to long-press on it as a result of ag/28270130, the label for it
should be "Lock Screen".

Fixes: 354872759
Test: manual - activate Talkback on foldable and udfps devices
Flag: EXEMPT bugfix
Change-Id: I7ce96664b0c88546776466a43ea113437fcda1f4