[INTERNAL] moved encoding attribute to FileActivity

TODO: Update the FileList. Also update the ProjectDeltaVisitor to
support this change.

Change-Id: I4ce673475d9c14c0b04ffbfc21e0d2a68d813b4f
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1742
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>