Polish predictive back with 3-button-nav

This slightly change spring stiffness of progress animation and scales Dialogs to the center.

Bug: 373544911
Test: BackAnimationSpecTest
Test: Manual, i.e. test sysui dialog predictive back and verify spring stiffness polish
Flag: com.android.window.flags.predictive_back_three_button_nav
Change-Id: I14556761bbcfe71421473ad881d01c2b9999a829