Flexiglass: fix occasional squished notifs on lockscreen

We were skipping one updateStackPosition() call to avoid flicker, but at
least occasionally, we were skipping *necessary* calls and ending up
with a mis-sized stack and squished notifications as a result.

Fixes: 352281989
Test: manual: see b/352281989#comment2 for repro steps
Flag: com.android.systemui.scene_container
Change-Id: I3c23abe7243a41176f6038be55a3ff08cf2e5c4d