Fix one-icon shelf flicker after lockscreen swipe-down-and-let-go

How the flicker happens:
- As we return to lockscreen after swipe down, fractionToShade
decreases, so sectionGap decreases.
- In NSSL#setFractionToShade, we don't immediately update
stackHeight with the smaller sectionGap, so the requested
children update works with the bigger stackHeight from before.
- However, StackScrollAlgorithm#updateChild uses the
updated (smaller) sectionGap, resulting in more room such that
the last notification shows partially above the shelf.
- The shelf no longer has a full view so it hides.

This change adds the missing call to updateContentHeight
after fractionToShade changes.

Bug: 222123657
Test: NotificationStackScrollLayoutTest
Test: on lockscreen, have one icon in shelf,
      swipe down and let go
      => shelf and last notification do not flicker
Change-Id: I59a6564b73c1acf15a046db403a185bdaaa5fafe