Convert GroupTask and DesktopTask to Kotlin

This is the first step in refactoring GroupTask to remove the assumption
about `task1` being always present.

Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: Ib79d9f2ad7f028b971e2509d846b50fa220bee26

quickstep/src/com/android/launcher3/taskbar/KeyboardQuickSwitchController.java

@@ -269,7 +269,7 @@ public final class KeyboardQuickSwitchController implements
         DesktopTask desktopTask = findDesktopTask(tasks);
 
         if (desktopTask != null) {
-            mTasks = desktopTask.tasks.stream()
+            mTasks = desktopTask.getTasks().stream()
                     .map(GroupTask::new)
                     .filter(task -> !shouldExcludeTask(task, taskIdsToExclude))
                     .collect(Collectors.toList());