Adds (1) Enable2gPreferenceController, the controller for the "Enable 2G" toggle, and (2) Enable2gPreferenceControllerTest, the corresponding unit tests. Test: manul & atest -c Enable2gPreferenceControllerTest Bug: b/163168917 Change-Id: Ib827cfcfd96fdea77b586c92ae0ec5b2bd5818c3