Migrate UserDictionaryList to DashboardFragment

- Build a controller to generate/manage a list of preferences.
- Move some logics to the controller and add tests.

Test: manual
Test: make RunSettingsRoboTests -j
      atest UniquePreferenceTest SettingsGatewayTest
Change-Id: Ia3d885cb8917c7d5498b87818e24b938f0d95dbb