[Settings] replace isNonTtyOrTtyOnVolteEnabled() in VoLTE

1. Code refactor
2. Replace ImsManager#isNonTtyOrTtyOnVolteEnabled() by combination of
   TelecomManager#getCurrentTtyMode() and
   ImsMmTelManager#isTtyOverVolteEnabled()

Bug: 140542283
Test: m RunSettingsRoboTests -j ROBOTEST_FILTER=Enhanced4gBasePreferenceControllerTest
Test: m RunSettingsRoboTests -j ROBOTEST_FILTER=Enhanced4gLteSliceHelperTest
Change-Id: I5ea5978529a7ae9a63161caca22fb29102f24150