Implement base functionality for flexible 2-app split

This CL changes the way splitscreen works behind the enableFlexibleTwoAppSplit flag.
- Divider snap points are now roughly at 30% and 70%, instead of 35% and 65%
- Resizing splitscreen will now push one app partially off the screen
- Tapping/focusing on the offscreen app will bring the offscreen app back onscreen, pushing the other app offscreen
- When apps are pushed offscreen, the onscreen app is now automatically focused

Additional polish/fixes needed:
- Currently, setting the ratio too small (20:80 or 10:90) results in a black screen glitch where part of the app window is obscured. (That's why it's only set to 30:70 in this patch).
- Measurements and animations need to be aligned to spec
- Overview tile may need a new treatment in the offscreen app case
- There is a small status bar overlap glitch when apps are offscreen

I verified:
- Nothing changes when flag is off
- When flag is on, there are some polish issues, but no known crash cases

Bug: 349828130
Test: When flag is off, behavior is unchanged and continues to pass tests
Flag: com.android.wm.shell.enable_flexible_two_app_split
Change-Id: I11bcdcb746c449905c40488831942b3180b1b77c