Fix action buttons visibility in handheld devices with blur enabled(1/n)

Added scrim in overview over blur.

Bug: 404479758
Test: Manual. Enter overview from home with and without tasks in handheld devices.
Flag: com.android.launcher3.enable_overview_background_wallpaper_blur
Change-Id: If8cb37099f2e2b1fa8bed1611ed359b49f255d64