Remove the special logic of the divider bar z-ordering

Remove the divider bar z-ordering logic since the divider bar is
attached to a single-top root task for split screen now. Remove
split anchor too because it is for legacy split screen using.
Also removed corresponding tests which are no longer needed after
this patch.

Bug: 199236198
Test: pass existing tests
Change-Id: I3435a3c81de78804c8110eb38bbacd0a23b391ef