[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
`SetDirectory` seems to be static. you can declare `nativeSetDirectory` as static method at java side. And here also be static of `XWalkPathHelper`. Then no need to create XWalkPathHelper instance.