Remove HFP Client Connection Service's usage of BluetoothHeadsetClient

Currently, the HFP Client Connection Service uses the
BluetoothHeadsetClient profile proxy to get the HFP Client Service to do
things like accept calls, hang up, etc. This proxy exposes a surface we
don't need to expose outside of the Bluetooth stack, and causes calls to
leave the stack and come back.

The HFP Client Connection Service is now a singleton started by the HFP
Client Service itself. The HFP Client Service is also a singleton that
we can get an instance of to invoke functions directly against.

This change makes the connection service invoke things directly against
the HFP Client Service.

Tag: #refactor
Bug: 206035301
Test: atest BluetoothInstrumentationTests
Change-Id: Ic7d003108f1b8ca8d83b77a12b10190621a92769