Remove nested ensureActivitiesVisible from keyguardGoingAway

By consolidating all of the calls to keyguardGoingAway for each display
into one inner loop.

Although this makes it harder to have individual going-away states for
each display via keyguardGoingAway calls, this is in line with our plan
to eliminate the going-away state and have transitions generated via
setKeyguardShowing(locked, aod).

Bug: 423613749
Test: atest KeyguardLockedTests
Test: atest DisplayContentTests
Flag: EXEMPT bugfix
Change-Id: I147c039411d92eb72d388da6100c6fe4aea1dd31