Fix bundle header background during expansion

This CL tackles two bugs:
1) The backgrounds of groups within bundles and the bundle background
were overlapping while the bundle is expanding. (only visible with
notificationRowTransparency() on)
2) There was a sync issue between the view system and Compose where
the header background would flicker for one frame when finish expanding.

Syncing Compose and View is rather complicated and this turned out to
be a simple fix instead. We always let the bundle show the header
background and let the children show their own background when
expanding (first issue).

Bug: 389839492 416520241
Test: Verified that the flicker is gone and during expansion none
of the backgrounds overlap.
Flag: com.android.systemui.notification_bundle_ui

Change-Id: I298f61649ead8e841c798a9b6ab67092be408faf