Ensure that DebuggerWindow is shown on headless systems.

- on headless systems, user-0 is invisible. And hence windows started by
  user-0 process are not visible.

Test: atest AmStartOptionsTest#DashD
Bug: 294279968
Fix: 302587507
Change-Id: I82c22d88ef58ad264d4156c3fc113b2ae57e2631