Fix split black when enable in landscape

We assume dismissing side should be DOCKED_INVALID when split
active, however, if snap algorithm only have one target, it will
get other side due to current algorithm.

Fix this by correct algorithm and ensure dismissing side be
DOCKED_INVALID when split active or layout.

Fix: 203745267
Test: manual
Test: pass existing tests
Change-Id: I077a93472dad89bc620e490dcae5170e9a56279b