Add missing import to fix access to R

Author: h3x4d3c1m4l

android/src/main/java/com/pdftron/pdftronflutter/helpers/PluginUtils.java

@@ -1,5 +1,7 @@
 package com.pdftron.pdftronflutter.helpers;
 
+import com.pdftron.pdftronflutter;
+
 import android.content.ContentResolver;
 import android.content.Context;
 import android.graphics.Color;