Fix Android Wikipedia app java checkstyles

- Use checkstyle-aggregate rather than checkstyle. This makes
  it easier to run just checkstyle on both the subprojects
  without having to actually build them
- Set encoding to utf-8 to prevent warning
- Look specifically for checkstyle-result.xml files only to
  prevent errors caused by presence of other checkstyle-* files.
- Use checkstyle publisher directly rather than the checkstyle-xml
  macro.

Change-Id: I38ae6767c4cac6a2857b3eae76b61d616b92de11