Security fix for VPN app killable via lockscreen.

Do not show the active apps dialog when the screen is locked.
Instead prompt the user to unlock directly if clicked and only open the dialog on
successsful unlock.

Merged-In: I384699d478e5abcee3a165afc45211b9ed96334a
Change-Id: I071cd9b7d0e779a156795d6af481231c5347101d
Flag: NONE
Bug: 304772709
Test: manually, locked and unlocked
Test: atest com.android.systemui.qs.FooterActionsControllerTest