Add aconfig flag for wallpaper change receiver registration in bg.

This flag will be used to decide whether broadcast receiver registration
will happen on main thread or bg executor.

Test: manual flash and wallpaper change
Bug: 327315860
Flag: com.android.systemui.register_wallpaper_notifier_background
Change-Id: I149447a2df750740bd8f78777f4f51503f0a6019