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

libs/WindowManager/Shell/src/com/android/wm/shell/transition/DefaultMixedHandler.java

@@ -150,9 +150,9 @@ public class DefaultMixedHandler implements Transitions.TransitionHandler,
     @Override
     public WindowContainerTransaction handleRequest(@NonNull IBinder transition,
             @NonNull TransitionRequestInfo request) {
-        if (mPipHandler.requestHasPipEnter(request) && mSplitHandler.isSplitScreenVisible()) {
+        if (mPipHandler.requestHasPipEnter(request) && mSplitHandler.isSplitActive()) {
             ProtoLog.v(ShellProtoLogGroup.WM_SHELL_TRANSITIONS, " Got a PiP-enter request while "
-                    + "Split-Screen is foreground, so treat it as Mixed.");
+                    + "Split-Screen is active, so treat it as Mixed.");
             if (request.getRemoteTransition() != null) {
                 throw new IllegalStateException("Unexpected remote transition in"
                         + "pip-enter-from-split request");