[Flexiglass] Don't hide the footerview during shade expansion

Showing/Hiding the FooterView during the shade open/close aniamtions are
unexpectedly changing the stack height, and is causing visual bugs:
 - Jump cut in the DualShade panel's height
 - Weird FooterView reveal animation in Single- and SplitShade.

Fixes: 438708057
Test: manually verify the shade open transitons with Single- Dual-
and SplitShade:
       - open shade via GlobalActions
       - open shade via Swipe
       - open shade via Fling
Flag: com.android.systemui.scene_container

Change-Id: Iaf47f4034a799031ff78ac8c52f20ab2e93165ad