Set an explicit pane title for communal hub.

This should fix a bug where TalkBack sometimes announces the wrong title
when entering communal hub.

Bug: 360753713
Test: Manually by enabling TalkBack and using actions from the lock
screen to enter communal hub.
Flag: EXEMPT bugfix

Change-Id: I880eae4a7ac5e6d7abae8f76ce2fb657305798bc