Device config for RTT and PhoneAccount capability

Add the device config flag for RTT (default false). Check for it and the
corresponding carrier config in order to set the RTT capability on the
telephony PhoneAccount.

Test: boot device and check for the capability
Bug: 63934808
Change-Id: Icdbe0c0124fbb7ac7f473739d311589a52feccb9
2 files changed