Fix WindowOrganizerTests for DisplayArea#isAttached

Before, the tests passed becasue DisplayArea#isAttached always return
true. It will be changed to check if it is attached to the
RootWindowContainer after ag/14950060. Update the tests first.

Bug: 183993924
Test: atest WmTests:WindowOrganizerTests
Change-Id: I2e5c16cd4508f03d2de13bf7bdca5fe748a6cd9a