Use SNAP_POSITION_KEY with null values

This removes the workaround introduced in ag/33657627, now that the
root cause is fixed.

Note that this also sets the `snapPosition` of the first and last zone,
which is different to the original CL.

Test: Manual
Bug: 383631946
Flag: com.android.wm.shell.enable_magnetic_split_divider
Change-Id: I1aaed55b7254e3a99507dc975491be4621d2156b