Allow SplitScreenController to explicitly request a new instance.

Adds a boolean parameter ignoreBackgroundTasks to startIntent to allow a
caller to skip the check for a background task that matches the new
intent. This is to fix an issue in desktop mode multi instance support
where the "New Window" option would instead fetch a background task.

Fix: 372969623
Fix: 372289869
Fix: 372269166
Test: Manual
Flag: com.android.window.flags.enable_desktop_windowing_multi_instance_features
Change-Id: I2284552c1253bb736c622da7523a0b3db36d2827