Fix missing initializartion of key preview position

This is followup of Iddab254c

Change-Id: I2d8e68b696779b742d29fdcf03c5fb509cb0bd87
diff --git a/java/src/com/android/inputmethod/keyboard/KeyboardView.java b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
index 1e7a061..f8bce40 100644
--- a/java/src/com/android/inputmethod/keyboard/KeyboardView.java
+++ b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
@@ -121,7 +121,7 @@
     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;