Consolidate entering split screen behavior

Add a fallback logic for entering split screen by verifying it contances
opening task to the indicated side to prevent leaving one side of the
split empty. Fix checking the wrong running task when entering split
screen by checking the top running task instead of checking reported
focusing task since the focusing task is not matching with the leaf task
running on top always.

Fix: 246689775
Fix: 260389099
Test: atest WMShellUnitTests
Test: drag and drop Calender to enter split while Calender on top won't
      leave empty side split.
Test: drag and drop Chrome to enter split while Chrome on top won't pup
      up not supported toast.
Change-Id: Ic7ea239b10ce41d956a85dbd6d04e007babd0743