Call getModelWriter() to get the newest version instead of using the old one

DropTargetHandler was calling getModelWriter() on a final variable
and the model writer it was using was and old one.

Fix: 280170665
Test: Manul test, delete widget and rotate the device, you have to do this twice.
Change-Id: I7b3ac4b272568fcc81438ffd5ac84cf7b64a62ea
diff --git a/src/com/android/launcher3/Launcher.java b/src/com/android/launcher3/Launcher.java
index 4764d72..29b0f08 100644
--- a/src/com/android/launcher3/Launcher.java
+++ b/src/com/android/launcher3/Launcher.java
@@ -1621,6 +1621,9 @@
         return mModel;
     }
 
+    /**
+     * Returns the ModelWriter writer, make sure to call the function every time you want to use it.
+     */
     public ModelWriter getModelWriter() {
         return mModelWriter;
     }