Minor cleanup of SnapTarget

This CL removes a unused field on the SnapTarget class, taskPosition, in preparation for flexible split.
Also streamlines 2 test cases on SplitLayoutTests in a way that I think makes more sense.

Bug: 349828130
Test: Behavior is unchanged and continues to pass tests
Flag: com.android.wm.shell.enable_flexible_split
Change-Id: I0f12391b8037d5966b37ff94af38d455edb4d725