Activity: Add parameter to bp_activity_get_comment_depth().
Previously, bp_activity_get_comment_depth() only supported looking for an
activity comment's depth during a queried activity comment loop.
This commit allows a developer to pass either a comment object or an
activity ID to the function so the activity comment depth can be fetched
outside the activity comment loop.
This fixes an issue with a notice displaying in the BuddyPress Activity
admin dashboard.
See #7329.