Fix the transparent group header

Fixing the transparent group header issue when the async header
inflation is enabled, dark mode just changed, and the group is
automate-generated. The new header’s mBackground was null before the
fix.

Fix: 340161724
Test: manual
Flag: com.android.systemui.notification_transparent_header_fix
Change-Id: I4f1bc2ec155a146013d5976d80c0dae10ddabb3f