Use task instead of sourceTask to detect the fullscreen launch

This CL fixes the usage of `task` vs `sourceTask` to detect the
fullscreen launch case.
In Ib48752887e7dfd685135c1e0b0e96cd6186708b2, the line unintentionally
uses `sourceTask` although it looks at the launch windowing mode of the
launching task itself (i.e., `task`).

Flag: EXEMPT - bug fix
Bug: 419348338
Bug: 416953378
Bug: 422035204
Bug: 419110280
Test: DesktopModeLaunchParamsModifierTests
Test: manual - Install a new app and launch it in desktop-first mode
Test: manual - Circle-to-search issue (b/410554758)
Change-Id: I2281ef0ec222219d891830d29959efe41a0160b8