Fix empty notif view flicker on keyguard

As the shade collapses, there is very slight gap between
when the shade collapses and status bar state changes to
KEYGUARD. During this gap, the empty notif view visibility
changes from VISIBLE->INVISIBLE->VISIBLE->GONE.

Ensure that the empty notif view alpha does not change
if the shade is collapsed.

Fixes: 333445519
Test: atest StackScrollAlgorithmTest
Flag: None
Change-Id: I16450015f13b5394e63f304d23f20e5e76da5d59