Root cause:
Only using and querying the dialer intent can't be represented for supporting RTT.
Next action:
Add one checker for carrier config.
Bug: 159242323
Test: manual test for AT&T, Google Fi, chunghwa sim
& make RunSettingsRoboTests ROBOTEST_FILTER=RTTSettingPreferenceControllerTest
Change-Id: I8e85b9fdfc658ea6855a4041691efdbfd07c62d6