Restore PiP to freeform only if Desktop Mode is active.

Recall: http://recall/clips/9b78d73d-6abb-41ef-a5a1-4fc6d93a6710

This is an update to I13006f479868f731550b9c75d8f9927bda2592fe such that
a PiP window will only expand/exit to freeform windowing mode if Desktop
Mode is currently active.

Bug: 376875483
Test: atest PipTaskOrganizerTest
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I317c4adcebdc33cddc1c04039728713cf7f4c45b