[Android] Copy missing file for XWalk library project after rebasing to 31.

* Copy ImageFormat.java to XWalk library project as it's added in version 31 and
it's essential.
* Change the API level to 18 in project.properties for Eclipse.
* This will require Eclipse/ADT to update the SDK to version 18.

BUG=
Merge pull request #846 from sqliu/rebase-for-library-project

[Android] Copy missing file for XWalk library project after rebasing to 31.