[Security Solution] Use authc.getCurrentUser from core.security in browser

[tsullivan]

follows https://github.com/elastic/kibana/pull/187004

Summary

This PR migrates the Security Solution Plugin's browser-side code that consume authc.getCurrentUser to use coreStart.security

Background: This PR serves as an example of a plugin migrating away from depending on the Security plugin, which is a high priority effort for the last release before 9.0.

Checklist

Delete any items that are not applicable to this PR.

• [x] Unit or functional tests were updated or added to match the most common scenarios