Fix RTT property setting

Don't start the RTT text reader until the IMS stack gives us a call
profile with RTT set to on.

Bug: 73832960
Test: manual
Change-Id: Ic85d16f602140d34c9680c3a1cb33e01b5cf02c7
Merged-In: Ic85d16f602140d34c9680c3a1cb33e01b5cf02c7