Return `RESULT_CONTINUE` in `DesktopModeLaunchParamsModifier`

Return `RESULT_CONTINUE` instead of `RESULT_DONE` in
`DesktopModeLaunchParamsModifier`. This allows the default modifer
(`TaskLaunchParamsModifier`) to also have an influence on the launch
params and populate any fields that are required. The
`TaskLaunchParamsModifier` honours the values set by previous modifers
so we do not have to worried about anything gettings overridden.

Fixes: 338076897
Fixes: 336998072
Test: atest WmTests:DesktopModeLaunchParamsModifierTests
Test: atest CtsWindowManagerDeviceMultiDisplay:MultiDisplayPolicyTests
Test: atest CtsWindowManagerDeviceMultiDisplay:MultiDisplayClientTests
Test: atest CtsWindowManagerDeviceMultiDisplay:MultiDisplayActivityLaunchTests

Change-Id: Iffb29064711bba99f7dee7121a2a4f1cc1c82f47