[Android] Re-enable NativeFileSystem instrumentation test

Root cause:
Instrument test loads test page as "Data URI scheme". Such type of URI
dosen't have URL origin. An empty URL origin will cause crash.

Fix:
Give a default value, "file:///", to URL origin to avoid crash.

Related to XWALK-99
Test case loads content like this "data:text/html,%3Chtml%3E%0A%20%20%3Chead%3E%0A%2" , not "file:///android_asset/native_file_system_api_test.html". That's called "Data URI Scheme", I mentioned above. Test framework reads the html file as string. Then create a Data URI from that string and send to xwalk. So, the context_url is "data:text/html, xxxx" rather than "file://xxxx".