Use separate SemanticKey for keys with null values

* Using the same key for all the segments resulted in
always returning null whenever we did motionValue.get(key)
* My guess is that the identitiy section we set after each
section using the same key was overwriting the segment section
* Either way, now we get valid snap targets when we query the
motion spec

Bug: 383631946
Flag: com.android.wm.shell.enable_magnetic_split_divider
Test: Manual
Change-Id: Iec9a6c269df87b3a913076ac60184398560089fd