Use disabled summary when trustagents are disabled

This makes trust agent preferences follow the behavior of other features that can be disabled by device admins, otherwise the preference is simply greyed out without context until the user clicks on the preference.

Bug: 319095039
Test: Covered by TrustAgentListPreferenceControllerTest#onResume_restrictedPreferenceShouldUseAdminDisabledSummary
Test: atest SettingsRoboTests:com.android.settings.security --host
Test: manually validated that disabled admin policy results in showing "Controlled by admin" as the summary
Flag: EXEMPT bugfix for single preference in settings
Change-Id: I89f052cf2479a120d8b5b223414162f129cde89e