Rely on presense of divider target to determine split screen state

* Don't rely on the number of leashes, since an app with
assistant invoked returns multiple remote app targets
even though the device isn't in split screen

Fixes: 197293347
Test: Repro steps in bug don't cause crash.
Less fatal bugs need to be addressed,
TODO(b/197568823)

Change-Id: I3432e3d8c54a433dd38d297db73ea3d46bad7ba9