Fix pip or bubbles is under split divider

In stage split, it always set the divider layer to top and it would
Cover some layer it should not cover such as pip or bubbles.

In legacy split, this ordering is handled by TDA but it cannot apply
on stage split because it directly use WindowlessWindowManager without
shell root.

Fix it by simplify the layer ordering. Make divider layer as a fixed
value. Normal task layer should always lower than it and always on top
tasks layer count start from it so we can ensure always on top tasks
higher than the divider.

Also move some constants layer value to WindowManagerPolicyConstants
for managerment and accessed by both wm-core and wm-shell.

Fix: 194359996
Test: open a pip app and active stage split then drap the pip to
      divider.
Test: Pass existing tests.
Change-Id: I6e4002435be350872aec5fa154849ed6e9561ce0