Update back gesture zone any time the screen size changes.

Fix: 336697780
Test: Manual testing, open widget picker on folded front screen and then unfold, the widget picker should be showing and the backgesture should work.
Flag: EXEMPT bugfix
Change-Id: Ie4a7bffbd9b24ba26826f825a7c66216f565952f

src/com/android/launcher3/Launcher.java

@@ -771,6 +771,7 @@ public class Launcher extends StatefulActivity<LauncherState>
             // initialized properly.
             onSaveInstanceState(new Bundle());
             mModel.rebindCallbacks();
+            updateDisallowBack();
         } finally {
             Trace.endSection();
         }