Fix AssistantStackTests for device with created-by-organizer root tasks

- Skip checking windowing mode as created-by-organizer root tasks can
  override windowing mode.
- Since getRootTaskByActivityType won't return created-by-organizer
  root tasks, the children task size can be 1 or 0.

Bug: 294250993
Test: android.server.wm.AssistantStackTests
Change-Id: I51fc4f359cf91c935c99cb0de23ecf3130b20d8e
Merged-In: I51fc4f359cf91c935c99cb0de23ecf3130b20d8e