Shade should be blurred only when shade blur flag is on

Other changes:
 - Fixes a couple of occlusion scenarios where the shade is open.

Bug: 370555003
Fixes: 391409919
Flag: com.android.systemui.notification_shade_blur
Test: updated unit tests, verified manually.
Change-Id: I21b683aa46c0286d270276225983b1554d217d80