Removed onBackPressed handling in SplitSCreenSelectState

- So it'll have exact same behavior as OverviewState to exit overview when going back
- Also removed RecentsView.cancelSplitSelect as it's longer needed

Fix: 181707736
Test: manual
Change-Id: Ide4dfc64680ecc9adfe245ae2de1463735b0490e

quickstep/src/com/android/launcher3/uioverrides/RecentsViewStateController.java

@@ -92,7 +92,6 @@ public final class RecentsViewStateController extends
             builder.add(mRecentsView.createSplitSelectInitAnimation().buildAnim());
             mRecentsView.applySplitPrimaryScrollOffset();
         } else if (!isSplitSelectionState(toState) && isSplitSelectionState(currentState)) {
-            builder.add(mRecentsView.cancelSplitSelect(true).buildAnim());
             mRecentsView.resetSplitPrimaryScrollOffset();
         }