[Android] Fix crash issue caused by the download listener.

This is because the XWalk core uses the request API to download
local file in the case, which would cause the crash.

So when downloading the local file, download listener would
only do the file transfer for this.

BUG=https://crosswalk-project.org/jira/browse/XWALK-338
Merge pull request #8 from huningxin/pick_download_fix

[Android] Fix crash issue caused by the download listener.