[Settings] replace isNonTtyOrTtyOnVolteEnabled() in WFC

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

Bug: 140542283
Test: manual
Change-Id: Ia5179ee1a23f054899bc74c796542bae40cdc8d5
8 files changed