Add correct offsets to offscreen tasks in flexible split

This CL makes it so that task offsets are added when apps go offscreen in flexible split. We do this to avoid a config change/app redraw while the task is temporarily outside the screen's bounds.

Fixes an issue where the config would constantly recalculate when shifting between focused apps in flexible split.

There is still a small issue with the navigation bar causing a small shift, which is a KI and we should fix in another patch.

Fixes: 401087186
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Tested flexible split in phone and tablet modes, and verified that the app doesn't constantly redraw anymore. Also thoroughly tested launch adjacent and app pair launch to make sure that they aren't affected.
Change-Id: Ic8ddb91e7b877b847a31ffc17be0fbf37fe53fb7