[Android ext] Fix bad comment in code example

The previous comment described the return value of the functions
incorrectly: the return value is actually a response object
which contains the "files" property (not just an array of files).

Partially fixes https://crosswalk-project.org/jira/browse/XWALK-1961
Merge pull request #125 from crosswalk-project/android-ext-fix-1961

[Android ext] Fix bad comment in code example