Switch from group background to children backgrounds when group expansion finishes.

Only do this for child rows not customizing its background color.
Will address the smooth fading of custom color backgrounds in follow up.

Bug:398951935
Test: atest ExpandableNotificationRowTest
Test: Expand a notification group, by clicking the expander or swiping.
Make sure there are no flickers.
Flag: com.android.systemui.notification_row_transparency

Change-Id: Id2cb41f7772efbf738ab155c6a99d8799f54b15d