[wm]: Support free snap mode

In aosp, split divider can be snapped to the specific targets
(first, middle, or last)

new concept is freely snapping between non-dismiss targets.

Test: manual test
1. change config_dockedStackDividerFreeSnapMode true in config.xml
2. build
3. make split state
4. drag divider any position

Bug: 123693029
Change-Id: I9fc42fb698659608de6509acc9ad44874e793fd5