[API] Adds FollowingActivitiesManager

This class is a 1:1 copy from Eclipse.

Change-Id: Ie80df9ea15ab26c2767fe918f369163e2904d22b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1926
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Holger Schmeisky <google@schmeisky.com>