Remove IActivityTaskManager#setTaskWindowingModeSplitScreenPrimary

It is the same as setTaskWindowingMode(SPLIT_SCREEN_PRIMARY).

Still keep ActivityTaskManager#setTaskWindowingModeSplitScreenPrimary
that redirects to setTaskWindowingMode for simple usages.

Remove SPLIT_SCREEN_CREATE_MODE_TOP_OR_LEFT and
SPLIT_SCREEN_CREATE_MODE_BOTTOM_OR_RIGHT because they are no longer
used since commits bed2d1b5, 4067a2d and 51beb146. Now if there needs
the similar functionality, it should be done by organizer in shell.

Remove the usage getNavBarPosition from StatusBar because it was
used to determine the split side.

The history about the removed fields: 59a73ca3, f59a4132.

Bug: 174040691
Test: SplitScreenTests
Change-Id: Ie5770d72cc5fb2f6ba1beceec418f1ea39f9c604