Adding a mock for MSDLPlayerWrapper to DeleteDropTargetTest.

The mock makes sure we don't rely on a device hardware for tests.

Test: DeleteDropTargetTest
Flag: com.android.launcher3.msdl_feedback
Bug: 379958544
Change-Id: I202b81f3dd16af1c0985980d0b962be0000bf31f

src/com/android/launcher3/ButtonDropTarget.java

@@ -65,7 +65,7 @@ public abstract class ButtonDropTarget extends TextView
     protected final ActivityContext mActivityContext;
     protected final DropTargetHandler mDropTargetHandler;
     protected DropTargetBar mDropTargetBar;
-    private final MSDLPlayerWrapper mMSDLPlayerWrapper;
+    private MSDLPlayerWrapper mMSDLPlayerWrapper;
 
     /** Whether this drop target is active for the current drag */
     protected boolean mActive;
@@ -438,6 +438,11 @@ public abstract class ButtonDropTarget extends TextView
         return textHeight + getPaddingTop() + getPaddingBottom() >= availableHeight;
     }
 
+    @VisibleForTesting
+    public void setMSDLPlayerWrapper(MSDLPlayerWrapper wrapper) {
+        mMSDLPlayerWrapper = wrapper;
+    }
+
     /**
      * Reduce the size of the text until it fits the measured width or reaches a minimum.
      *