Expose blurs to SurfaceControl

SurfaceControl now has #setBackgroundBlurRadius, which receives a radius in pixels
pointing out how much the content behind the current window should be blurred.

Bug: 141640413
Test: visual; on power menu and shade
Test: adb shell dumpsys activity service com.android.systemui
Change-Id: I367f8a7199d5b37ad476d3c2bc02c8900f8c1f9e