Fix freeform task launch handling when keyguard locked

- Do not handle freeform task launches when keyguard is locked. The task
  windowing mode changes to fullscreen (More details: b/160925539).

Bug: 348276049
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Test: atest CtsWindowManagerDeviceActivity:ActivityVisibilityTests
Flag: EXEMPT bugfix
Change-Id: Ie219fbb2faef332739a5d2ca40a26634387f6680