Documenting that AccessibilityWindowInfo#getRoot() can return unimportant nodes

Updates documentation to clarify existing behavior.
Also refines AccessibilityInteractionControllerTest
to more specifically test the expected behavior.

Test: atest AccessibilityInteractionControllerTest
Flag: EXEMPT only documentation
Bug: 407099631
Change-Id: I1b5b1605575e956692425c5c86928a4594772bfc