Fix key preview centering given magnification

Author: yawnoc

app/src/main/java/io/github/yawnoc/strokeinput/InputContainer.java

@@ -682,7 +682,7 @@ private void updateKeyPreview() {
     final int previewHeight = keyPreview.height;
 
     final int previewMargin = activeKey.previewMargin;
-    final int previewX = activeKey.x;
+    final int previewX = activeKey.x - (previewWidth - activeKey.width) / 2;
     final int previewY = activeKey.y - previewHeight - previewMargin;
 
     if (keyPreviewPopup.isShowing()) {