Exit immersive on new instance launches

Adds missing immersive check during Shell-started new task instance
launches (e.g. from the "New Window" button in the app header). This
ensures that when launching a new instance over an immersive task, the
immersive task exits immersive and is resized back to its previous
bounds.

This change also changes the new instance transition handler to use the
Desktop Mixed handler, so that both the launching change and the
immersive change are animated nicely.

Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Fix: 377812444
Test: With Chrome and multi-instance desktop flag enabled, enter desktop
immersive, open a new window using the "New Window" button in the app
header and verify the immersive window exits and both windows animate.

Change-Id: I27110e3fc1984b513865bdac82b69511825b93e2