Modify callback to allow scroll handle toggling if onTapListener is not set

Improve javadoc a little bit

Author: barteksc

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/DragPinchManager.java

@@ -83,7 +83,7 @@ public void setSwipeVertical(boolean swipeVertical) {
     @Override
     public boolean onSingleTapConfirmed(MotionEvent e) {
         OnTapListener onTapListener = pdfView.getOnTapListener();
-        if (onTapListener != null && !onTapListener.onTap(e)) {
+        if (onTapListener == null || !onTapListener.onTap(e)) {
             ScrollHandle ps = pdfView.getScrollHandle();
             if (ps != null && !pdfView.documentFitsView()) {
                 if (!ps.shown()) {

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/listener/OnTapListener.java

@@ -18,16 +18,16 @@
 import android.view.MotionEvent;
 
 /**
- * Implements this interface to receive events from PDFView
- * when a page has been scrolled
+ * Implement this interface to receive events from PDFView
+ * when view has been touched
  */
 public interface OnTapListener {
 
     /**
-     * Called when the user has a tap gesture, before processing scroll toggling.
+     * Called when the user has a tap gesture, before processing scroll handle toggling
      *
      * @param e MotionEvent that registered as a confirmed single tap
-     * @return true if the single tap was handled
+     * @return true if the single tap was handled, false to toggle scroll handle
      */
     boolean onTap(MotionEvent e);
 }