Update INVALID_LAYER_STACK naming

INVALID_LAYER_STACK is renamed to UNASSIGNED_LAYER_STACK
for clarity since this LayerStack value is actually valid
and can be used to represent offscreen layers.

Bug: b/397775142
Test: atest SurfaceFlinger_test
Flag: EXEMPT, renaming
Change-Id: I72e19ca81891cce5569bd3299a21d8297a52d4fa