Polish back-to-home animation for 3-button-nav

Move window to the screen center when the animation is driven by a long press on the back button in 3-button-nav.

Bug: 373544911
Test: Manual, i.e. verify that app is center scaled
Flag: com.android.window.flags.predictive_back_three_button_nav
Change-Id: I8bacb923e3c81d99ddc1a8900bcc7423c8276c55