Fix bundled groups not expanding on long press

This bug only existed when there is exactly one child in the bundle.
This made the condition trigger that makes sure that when we swipe and
there is only 1 child we swipe the entire group away rather than just
the child.

The problem here is that we can't just modify the condition in
getChildAtPosition as I'm not aware how we could make this judgment
without also knowing which MotionEvent we are dealing with. For a
swipe the dismiss action should still target the parent when conditions
are met but for a longpress we want to target the child. This requires
us to introduce this new method getSwipeTarget() that can implement
specific behavior for ACTION_MOVE.

Bug: b/419100765
Test: Refactor only verified manually
Flag: com.android.systemui.notification_bundle_ui
Change-Id: Iff6370093a25c64a345eec0fe35fa965dc707ce4