Allow bp_parse_arg filters to be used on the WP-Admin Activities page.

[rekmla]

Currently, the function bp_activity_get() does not fully support bp_parse_args() since it does not pass the filter_key parameter.

It would be very helpful if this function passed 'has_activities' or some/any other value in the filter_key parameter of bp_parse_args() so that we can write a filter that works on the WP-Admin Activities page.

[boonebgorges]

Thanks for the suggestion! bp_activity_get() was converted to use bp_parse_args() in [8710], but the identifier was not properly added at that time.

'has_activities' is already in use by bp_has_activities(). Let's use 'activity_get'.

[boonebgorges]

In 11078:

Activity: Pass unique identifier to bp_parse_args() filter in bp_activity_get().

Props rekmla.
Fixes #7250.