Add carrier config key for TTY support.

Add carrier config key to determine if a carrier supports TTY.

Bug: 116764319
Test: Manual; turned off key and observed settings doesn't show TTY.
Test: Manual; turned on key (default) and observed settings does show TTY.
Change-Id: I9dff154740213ff9627c0cc07452f0b22b59e8f6
2 files changed