Lockscreen FSI HUN: collapse when space is low

In StackSizeCalculator, calculate spaceNeeded for both
- when there is enough space on lockscreen to always show
  heads up layout, and
- when there is not enough space (whereupon we always use the
  collapsed layout/minHeight, even for FSI HUNs that otherwise
  show the heads up layout)

Determine which result to use based on if we can fit at
least 1 notification without saving space.

In ExpandableNotificationRow, choose the layout based on if we are
saving space on lockscreen.

Bug: 272150926
Test: NotificationStackSizeCalculatorTest
Test: empty lockscreen => HUN layout
Test: lots of notifs and showing shelf => HUN layout
Test: UMO, enough space => HUN layout
Test: UMO, not enough space => collapsed layout
Test: Split shade UMO, enough space => HUN layout
Test: AOD (enough space) transition to UMO (not enough space)
      => HUN to collapsed layout transition

Change-Id: Ib78616528165206009367bd6ee86a328a1a66008