Cleanup the legacy split screen (1/n)

Android switched to using stage split since SC-V2, we can clean up
legacy split screens from the code base.

Bug: 199236198
Test: atest TaskTests RecentTasksTest DisplayPolicyTests
      ActivityStarterTests ActivityTaskSupervisorTests

Change-Id: I0454efaf6bf93a0c7ff2512efb8bd276ed3838f5