[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
```
private native void nativeSetDirectory(long nativeXWalkPathHelper,
        String virtualRoot, String path);
```
or
```
private native void nativeSetDirectory(long nativeXWalkPathHelper,
                                       String virtualRoot,
                                       String path);
```