[Flexiglass] Ignore lookahead pass for the notifications top callback

SingleShadeMeasurePolicy was reporting measurement results during both
the main and lookahead layout passes. This caused conflicting values for
the max scrim top calculation, which resulted in a visual flicker while
transitioning from Shade to Gone.

This CL makes sure that we're only using values from the main layout
pass, to provide a consistent value that reflects the current UI state.

Fixes: 437882570
Test: Slowly close SingleShade -> Observe the EmptyShadeView positioning
Flag: com.android.systemui.scene_container
Change-Id: Ic4ebb7147ea92670968ec28a86bda82049141a13