Skip task overlay in calcuating task description

- When a PiP task is dismissed with the PiP menu activity showing, we
  use the wrong task description (belonging to the menu activity) to draw
  the app theme snapshot. The task overlay is only used from SysUI to
  overlay an activity on top of the task, and should not be considered in
  the calculation for the task description for the task.

Bug: 77964409
Test: Manual, launch Maps in PiP, dismiss and then open overview

Change-Id: I2d537b9fdd19eb69a8dc20f15ac5b263584d2fad