fix to problem with updating APK in Google Play keeping old OBB (#17689)

* fix to problem with updating APK in Google Play keeping OBB from previous version

This patch fixes the problem when we are unable to keep OBB from previous version when we are uploading new .apk to Google Play.
It was discussed here:
http://discuss.cocos2d-x.org/t/problem-with-updating-apk-in-google-play-keeping-obb-from-previous-version-because-versioncode-changes/36039

* check for null pointer added

Found an issue which could lead to a crash because of null pointer exception.
If pathToOBB doesn't exist then File.list will return null which will cause fileNames array be null.
Now it is fixed.

Author: hellobody

cocos/platform/android/java/src/org/cocos2dx/lib/Cocos2dxHelper.java

@@ -55,6 +55,7 @@ of this software and associated documentation files (the "Software"), to deal
 
 import java.io.IOException;
 import java.io.File;
+import java.io.FilenameFilter;
 import java.io.UnsupportedEncodingException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
@@ -192,16 +193,23 @@ public static void init(final Activity activity) {
     public static String getAssetsPath()
     {
         if (Cocos2dxHelper.sAssetsPath == "") {
-            int versionCode = 1;
-            try {
-                versionCode = Cocos2dxHelper.sActivity.getPackageManager().getPackageInfo(Cocos2dxHelper.sPackageName, 0).versionCode;
-            } catch (NameNotFoundException e) {
-                e.printStackTrace();
-            }
-            String pathToOBB = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Android/obb/" + Cocos2dxHelper.sPackageName + "/main." + versionCode + "." + Cocos2dxHelper.sPackageName + ".obb";
-            File obbFile = new File(pathToOBB);
+
+            String pathToOBB = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Android/obb/" + Cocos2dxHelper.sPackageName;
+
+	    	// Listing all files inside the folder (pathToOBB) where OBB files are expected to be found.
+            String[] fileNames = new File(pathToOBB).list(new FilenameFilter() { // Using filter to pick up only main OBB file name.
+                public boolean accept(File dir, String name) {
+                    return name.startsWith("main.") && name.endsWith(".obb");  // It's possible to filter only by extension here to get path to patch OBB file also.
+                }
+            });
+
+            String fullPathToOBB = "";
+            if (fileNames != null && fileNames.length > 0)  // If there is at least 1 element inside the array with OBB file names, then we may think fileNames[0] will have desired main OBB file name.
+                fullPathToOBB = pathToOBB + "/" + fileNames[0];  // Composing full file name for main OBB file.
+
+            File obbFile = new File(fullPathToOBB);
             if (obbFile.exists())
-                Cocos2dxHelper.sAssetsPath = pathToOBB;
+                Cocos2dxHelper.sAssetsPath = fullPathToOBB;
             else
                 Cocos2dxHelper.sAssetsPath = Cocos2dxHelper.sActivity.getApplicationInfo().sourceDir;
         }