Fix missing initializartion of key preview position

This is followup of Iddab254c

Change-Id: I2d8e68b696779b742d29fdcf03c5fb509cb0bd87

java/src/com/android/inputmethod/keyboard/KeyboardView.java

@@ -121,7 +121,7 @@ public class KeyboardView extends View implements PointerTracker.UIProxy {
     private float mPreviewTextRatio;
     private int mPreviewTextSize;
     private boolean mShowKeyPreviewPopup = true;
-    private int mKeyPreviewPopupDisplayedY;
+    private int mKeyPreviewPopupDisplayedY = -1;
     private final int mDelayBeforePreview;
     private int mDelayAfterPreview;
     private ViewGroup mPreviewPlacer;