[Dual Shade] Blur Notifications when QS is open over LS

Blur the NSSL's content when it is displayed on the LockScreen, and is
covered by the QuickSettings overlay.

Given the limitation of Flexiglass, that the NSSL is always z-ordered
above everything, this CL overrides NSSL's draw phase so that it can
create the visual impression of opening the QS over Notifications by:
 - drawing everything (but HUNs) to a separate canvas with a blur effect
 - clipping the blurred content by the QS shape
 - drawing the HUN on top of everything without blur, without clipping

Bug: 388469101
Test: Open the QS over the LS, and reveive a HUN
       - have a few notifications on the LS
       - drag down from top-right to open QS shade
       - receive a HUN
Flag: com.android.systemui.scene_container
Change-Id: I18b264f5b13ea5335dab1b23a99dbe97944248e5