Add aconfig flag for fixing the transparent group header

Add aconfig flag for fixing the transparent group header issue when the
 async header inflation is enabled, dark mode just changed, and the
 group is automate-generated.

Bug: 340161724
Test: n/a
Flag: com.android.systemui.notification_transparent_header_fix
Change-Id: Iaf1a59e047b1a284fdd4f50be7811c519a48bac9