Check carrierconfig for whether to ignore RTT mode

When sending the RTT on/off flag to the modem, check the carrier config
setting for whether we should always keep it on regardless of the
user-provided setting

Bug: 136035164
Test: manual
Change-Id: I9018b2d4abad70a8be398a646e2ad184f17cea5a
Merged-In: I9018b2d4abad70a8be398a646e2ad184f17cea5a