[Android] Native File System API

Completed features list:

Create file
Read file
Write file
Remove file
Create Directory
List Directory Entries
Remove Directory

BUG=https://crosswalk-project.org/jira/browse/XWALK-672
8-space indent for above two lines. Seems following does not exceed 100 limit, if so I'd prefer
```
nativeSetDirectory(mXWalkPathHelper,
                   names[i],
                   Environment.getExternalStoragePublicDirectory(dirs[i]).getPath());
```