HFP: Disable BTRH feature

Fluoride does not notify application layer upon AT+BTRH, and always
sends error response (BTRH is not supported) to remote device.
That is, Fluoride does not support BTRH feature.
But BTRH is enabled by default, so OK is sent to remote device by mistake
before the error response is sent. This makes some carkits unstable.

Bug: 29587048

Change-Id: If1aa32291491982851ecd480c4ba8390f39a27a2