[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
I tried to set baseUrl and a virtualUrl of LoadUrlParam to "file:///android_asset/native_file_system_api_test.html" in XWalkContent::doLoadUrl() (Ln.164 XWalkContent.java). The context_url is empty as before. In Chromium,  "content", the second parameter of XWalkView::load(), is also passed as Url. "baseUrl" and "virtualUrl" cannot overwrite it. On other way, In IsolatedFileSystem::GetIsolatedFileSystem(), "baseUrl" and "virtualUrl" cannot be accessed. Therefore, The work round in current commit is needed.