Use isSplitActive vs visible when detecting mixed pip/split

split was changed to preserve splits in "background", so it
needs to be detected via isSplitActive instead of isSplitVisible
so that it can update properly when a PiP transition comes in.

Bug: 280253323
Test: wait for b/280523948, then open split with an auto-pip
      and then swipe-to-home. Try this with both single and
      multi-activity pip and dump window containers to verify
      correct hierarchy.
Change-Id: Id3f28cad593a799933ddc24a570b7a91e22bd275