doc: Add EchoCancelingNoiseReduction to Handsfree

doc: Add EchoCancelingNoiseReduction to Handsfree

This patch adds echo canceling and noise reduction property to Handsfree
interface, allowing the handsfree unit to disable this feature in the
audio gateway. The HFP spec defines the procedure to turn off echo
canceling and noise reduction only, it is not allowed to enable this
feature from the HF unit.

According to Bluetooth HFP 1.6 spec: By default, if the AG supports its
own embedded echo canceling and/or noise reduction functions, it shall
have them activated until the AT+NREC command is received from the HF
unit. From then on, and until the current Service Level Connection is
dropped for any reason, the AG shall disable these functions every time
an Audio Connection is established.