Fix trust agent summary not being updated.

- move the handling for enabling the trust agent preference and its
summary from displayPreference() into updateState(), so that when user
navigates to change screen lock or update trust agent and returns to the
security settings page, the trust agent preference enabled state and
summary will be refreshed correctly.

Change-Id: I60bccfdff755a046a9f5453d39a7ac9d3ce0277c
Fixes: 72420147
Test: make RunSettingsRoboTests