Do not force-show system bars in desktop's full immersive mode

Change the force-show policy when in freeform to only apply when the top
freeform task isn't taking full-screen bounds, to allow freeform to have
an immersive-like mode where the system bars can be hidden.

Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Bug: 369444183
Bug: 369444147
Bug: 369443876
Test: atest InsetsPolicyTest
Test: enter full immersive in desktop, verify system bars are hidden
Change-Id: Ic9022dcf31f4b513cea87cba323cf249ee238774