[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
This commit maybe not very related with this PR and won't block the merge.

How about added test to `CoreTest`, thus, `RuntimeClientTest` and `RuntimeClientEmbeddedTest` are not needed. @wang16 any idea?