Closes elastic#164083
Related to elastic/sdh-beats#3759
Related to elastic#157844

This PR addresses two current issues affecting agent selection in Fleet
UI:
1. When there are inactive agents that are not listed on the current
page and "Select everything on all pages" is clicked, the count of
actionable agents is incorrect (cf. [this
comment](elastic#164083 (comment))
for details). This can have two consequences:
1. Incorrect and sometimes negative agent count in the "Actions"
dropdown.
   2. Disabled menu items in the "Actions" dropdown.
2. The "Select everything on all pages button is incorrectly displayed
when there are agents on managed policies on the page and there is no
pagination (cf. [this
comment](elastic#164083 (comment))).

(cherry picked from commit 591df70)

# Conflicts:
#	x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_list_page/components/bulk_actions.test.tsx