Fix bug 2646531 - regression in home screen bitmap caching

Looks like the code path for buildDrawingCache(false) has some bugs.
This reverts to the old code path and tries to manage the creation of
those caches a bit better.

Change-Id: Ic468e9db396c51f723454dc3832e0cd1c0c82004

src/com/android/launcher2/CellLayout.java

@@ -584,7 +584,7 @@ public class CellLayout extends ViewGroup {
             final View view = getChildAt(i);
             view.setDrawingCacheEnabled(enabled);
             // Update the drawing caches
-            view.buildDrawingCache(false);
+            view.buildDrawingCache(true);
         }
     }
 

src/com/android/launcher2/Workspace.java

@@ -666,7 +666,7 @@ public class Workspace extends ViewGroup implements DropTarget, DragSource, Drag
                         // Scroll if the user moved far enough along the X axis
                         mTouchState = TOUCH_STATE_SCROLLING;
                         mLastMotionX = x;
-                        enableChildrenCache(0, getChildCount());
+                        enableChildrenCache(mCurrentScreen - 1, mCurrentScreen + 1);
                     }
                     // Either way, cancel any pending longpress
                     if (mAllowLongPress) {
@@ -868,6 +868,9 @@ public class Workspace extends ViewGroup implements DropTarget, DragSource, Drag
             // Remember where the motion event started
             mLastMotionX = ev.getX();
             mActivePointerId = ev.getPointerId(0);
+            if (mTouchState == TOUCH_STATE_SCROLLING) {
+                enableChildrenCache(mCurrentScreen - 1, mCurrentScreen + 1);
+            }
             break;
         case MotionEvent.ACTION_MOVE:
             if (mTouchState == TOUCH_STATE_SCROLLING) {