Allow split position constants to be used in launcher.

- Added SplitScreenConstants class and added this class to the exported files to allow exporting window manager constants and methods to the launcher.

Bug: 198438631
Test: manual
Change-Id: I7c5649449389413d1435798c3e757103c4ced038
Merged-In: I7c5649449389413d1435798c3e757103c4ced038
(cherry picked from commit a191eb7e3c099a3ffde25d59d5203f2b3e4bdfa2)