Fix NPE from StageLayout not being initialized

- The StageLayout is only created when the split root task is created,
  but the ShellDropTarget is added immediately when SysUI starts and
  there can be a race where the left/right split state is queried
  before the split root is fully created

Bug: 314089952
Test: Presubmit
Change-Id: Ie3ffcc4d87f4fbc692b901e6501c588b013b692a