Add footer in private space auto lock settings page

Add a footer note to private space auto lock settings page that apps in
private space may need to be authenticated unlock when a separate lock
is set for private space.

Screenshot:
go/ss/7fE8epun3A2hgq4.png

Bug: 343166689
Test: Manual
Change-Id: I18c650eba128da512116a3166babb49f7ef33bb3