Enable reading files from the assets directory.

The cdvfile scheme allows access to defined filesystems, but before this
commit, it wasn't possible to read files from application's assets folder.

An example URL to access the cordova.js file would be:
cdvfile://localhost/assets/www/cordova.js

README.md

@@ -496,7 +496,7 @@ Android recognize a <preference> tag in `config.xml` which names the
 filesystems to be installed. By default, all file-system roots are enabled.
 
     <preference name="iosExtraFilesystems" value="library,library-nosync,documents,documents-nosync,cache,bundle,root" />
-    <preference name="AndroidExtraFilesystems" value="files,files-external,documents,sdcard,cache,cache-external,root" />
+    <preference name="AndroidExtraFilesystems" value="files,files-external,documents,sdcard,cache,cache-external,root,assets" />
 
 ### Android
 
@@ -506,6 +506,7 @@ filesystems to be installed. By default, all file-system roots are enabled.
 * `cache`: The application's internal cache directory
 * `cache-external`: The application's external cache directory
 * `root`: The entire device filesystem
+* `assets`: The application's assets directory
 
 Android also supports a special filesystem named "documents", which represents a "/Documents/" subdirectory within the "files" filesystem.
 

src/android/AssetFilesystem.java

@@ -262,7 +262,7 @@ long truncateFileAtURL(LocalFilesystemURL inputURL, long size) throws IOExceptio
 
     @Override
     String filesystemPathForURL(LocalFilesystemURL url) {
-        return null;
+        return toNativeUri(url).getPath();
     }
 
     @Override

tests/tests.js

@@ -3632,7 +3632,7 @@ exports.defineManualTests = function (contentEl, createActionButton) {
     var fsRoots = {
         "ios" : "library,library-nosync,documents,documents-nosync,cache,bundle,root,private",
         "osx" : "library,library-nosync,documents,documents-nosync,cache,bundle,root,private",
-        "android" : "files,files-external,documents,sdcard,cache,cache-external,root",
+        "android" : "files,files-external,documents,sdcard,cache,cache-external,root,assets",
         "amazon-fireos" : "files,files-external,documents,sdcard,cache,cache-external,root",
         "windows": "temporary,persistent"
     };