Transition out of split screen when triggering enter desktop.

In its current state, the split to desktop transition did not exit split screen, causing an issue where both tasks would close when attempting the transition. This CL prepares the task to exit split and closes the other task when performing the transition.

Bug: 296926062
Test: Manual; do split to desktop transition via both dragging and
caption menu button.

Change-Id: I034f9c677d3bbf280fed2e5dbd7f910f02789fdb