Ensure SAVE_APP_PAIR is only used for GroupedTaskView

- Check if TaskView is GroupedTaskView before casting to avoid a crash, caused by DesktopTaskView.containsMultipleTasks() is true but is not a GroupedTaskView

Fix: 331226084
Test: With Talkback on and Desktop task, go to Overview
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Change-Id: Ic2f1b940bb5a6b0cf452465b9cc15ebf4c256858