Added Gravity.CENTER_HORIZONTAL to drop target layout params.

I'm not sure if it is strictly necessary, but it is probably good to
be more explicit regardless.

Change-Id: Ib7647dbcb5449e22ef87da8d49b4e7804ba5c17f

src/com/android/launcher3/DeviceProfile.java

@@ -582,7 +582,7 @@ public class DeviceProfile {
             targetsLp.height = LayoutParams.WRAP_CONTENT;
         } else {
             // Horizontal drop target bar space
-            lp.gravity = verticalGravity;
+            lp.gravity = verticalGravity | Gravity.CENTER_HORIZONTAL;
             lp.height = searchBarSpaceHeightPx;
         }
         return lp;