Blur the hub when shade is showing

This change makes the hub blurred when shade is open. Because the
current blur is a window blur, and the hub lives in the same window as
the shade, it needs to be manually blurred. This is similar to the
manual blurring when bouncer is opened.

It also updates the alpha of scrim_behind when hub is showing to match
keyguard.

Test: atest CommunalViewModelTest
Fix: 410071710
Flag: com.android.systemui.notification_shade_blur
Change-Id: Iafadd592141e1fe072c57e8bf09ea08253af3e25