Activity: Conditionally load action and screen functions.

This commit conditionally loads action and screen function code for the
Activity component, utilizing the 'bp_late_include' hook introduced in
r11884.

Previously, we loaded these functions at all times, which is unnecessary
when a user is not on an BuddyPress activity page. Now, we only load this
code when needed.

A special case is required for PHPUnit as PHPUnit doesn't like conditional-
included files when running multiple unit tests. So for PHPUnit, we
include all action and screen code at once, just like before.

See #7218.