TrustAgentSettings use DashboardFragment

- Remove dummy preference from trust_agent_setting.xml.
- Build a controller to generate/manage a list of preferences.
- Move some logics to the controller and add tests.

Bug: 73899467
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.security
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.core
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.search
Test: atest SettingsGatewayTest UniquePreferenceTest
Change-Id: Ideae0c1e7311d7647cf522e01592822e565a0ff7