Fix TaskViewTouchController crash in seascape

Previously, we were using scroll direction POSITIVE as a catch all
to mean "up" but in seascape, we actually want NEGATIVE. Added
getUpDirection() to capture that. Tried to clarify the code a bit
by putting all the methods used solely by TaskViewTouchController
together with documentation. It's still pretty confusing and feels
redundant, but couldn't think of an obvious way to simplify.

Test: Swipe up and down on a task in all permutations of:
- 3 button mode
- Gesture navigation
- Portrait
- Landscape
- Seascape
- LTR
- RTL
- Home rotation allowed
- Home rotation disallowed

Fixes: 174009771
Fixes: 173567204
Change-Id: Id0f8d6f4365d888eb46182d8544d18206795dfb8

src/com/android/launcher3/touch/SingleAxisSwipeDetector.java

@@ -60,6 +60,11 @@ public class SingleAxisSwipeDetector extends BaseSwipeDetector {
             return direction.x;
         }
 
+        @NonNull
+        @Override
+        public String toString() {
+            return "VERTICAL";
+        }
     };
 
     public static final Direction HORIZONTAL = new Direction() {
@@ -86,6 +91,11 @@ public class SingleAxisSwipeDetector extends BaseSwipeDetector {
             return direction.y;
         }
 
+        @NonNull
+        @Override
+        public String toString() {
+            return "HORIZONTAL";
+        }
     };
 
     private final Direction mDir;
@@ -117,10 +127,6 @@ public class SingleAxisSwipeDetector extends BaseSwipeDetector {
         mIgnoreSlopWhenSettling = ignoreSlop;
     }
 
-    public int getScrollDirections() {
-        return mScrollDirections;
-    }
-
     /**
      * Returns if the start drag was towards the positive direction or negative.
      *