Explicitly use main dispatcher for the window root view blurs

The plan is to submit this CL and then move the flag to staging again, get new jank metrics and see how much it improves jank compared to the previous flag flip.

Bug: 392240569
Flag: com.android.systemui.bouncer_ui_revamp
Test: NA, everything builds, blur works.
Change-Id: Ie6de9b15b7291dbc5b07e4fb417e1529ad0fcb36