[3/N] Desks: Adjust root task opacity calculation

Changes how non-leaf container opacity is calculated. A container was
considered opaque if it contained any opaque activity within it. Now, to
support container hierarchies that might contain an opaque activity but
are still see-through (such as when an opaque activity/task does not
fill its parent), the calculation is changed to require that at least
one of its children is opaque and matches its bounds.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: with a hierarchy such as:
  (front)
  - Freeform root (with fullscreen bounds)
  -- Freeform child (non-fullscreen bounds)
  - Fullscreen task
The fullscreen task remains visible/resumed along with the freeform
root/child
Test: split-screen occludes (makes invisible) the task behind it

Change-Id: I166fa4977d6c1e988275d58ea718ce1119ab9b66