Pass in end snap position in EnterSession for entering split screen

* Depending on the end layout, we can determine which app needs
to be in focus. For flex split it should always be the larger one

Fixes: 396026394
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Back gesture always goes to 90 app. For 50/50 split back
gesture behavior unaffected.

Change-Id: Id9218e21d6335eebc6243722ebd057b42c096c51