Move primary-split-screen stack forward in some cases.

If we are moving a stack in split-screen seconardy mode forward, we
need to make sure we move the primary split-screen stack forward in
the case it is current behind a fulscreen stack so both halves of the
split-screen appear on-top and the fullscreen stack isn't cutting
between them.
Mostly a workaround until fix can fix b/70677280 where we would have
a stack as a parent of another stack.

Change-Id: I7d030b80c12771e0370a1fea8e1abf96560a9029
Fixes: 71899050
Bug: 70677280
Test: atest ActivityStackTests#testSplitScreenMoveToFront