Do not send AT+CHLD=? if the 3-way call feature is not supported

The HF shall not issue the AT+CHLD=? test command in case
either the HF or the AG does not support the Three-way calling feature.
Currently HF sends AT+CHLD=? to AG device when SLC sequence on going.
It affects to SCO statemachine so that after going out of range and
coming back in range while active call alives, SCO state goes bad and
never comes back. This is out of specification of HFP and it should be
fixed by checking peer and local 3way call capability check.

Bug: 25703926
Change-Id: I66adac2345c6fb0df6741fdbfa67d9483fc38a00