Fix persistent caching for BP_Activity_Activity::get_activity_comments()

- Use the 'bp' cache group consistently
- When a query shows that an activity item has no comments, do not cache the
  boolean value false. This will cause a cache miss the next time around.
  Instead, store the string 'none', and then convert it to false when
  requerying.

Fixes #5369

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7773 cdf35c40-ae34-48e0-9cc9-0c9da1808c22