Use Launcher.getLauncher(Context) instead of type-casting.

Bug: 32010039
Change-Id: Icb80d3e547a706740b84df2cb80f99e4a7f81c43
diff --git a/src/com/android/launcher3/ButtonDropTarget.java b/src/com/android/launcher3/ButtonDropTarget.java
index cf8abae..60a2cc3 100644
--- a/src/com/android/launcher3/ButtonDropTarget.java
+++ b/src/com/android/launcher3/ButtonDropTarget.java
@@ -83,7 +83,7 @@
 
     public ButtonDropTarget(Context context, AttributeSet attrs, int defStyle) {
         super(context, attrs, defStyle);
-        mLauncher = (Launcher) context;
+        mLauncher = Launcher.getLauncher(context);
 
         Resources resources = getResources();
         mBottomDragPadding = resources.getDimensionPixelSize(R.dimen.drop_target_drag_padding);