HFP Audio client - fix sink/source naming

In HFP software encoding/decoding data path, the encode path
reads data from audio sink, encodes the data and then sends to SCO.
The decoding path receives data from SCO, decodes the data and then
writes to audio source.

This change fixes the naming in HFP client. The affected classes and
functions don't have real users at the moment.

Added unittest to cover the read/write operations in HFP client
interface.

Bug: 349290628
Test: atest --host bluetooth-test-audio-hal-hfp-client-interface
Change-Id: I907015f62f2dacecaed8ebc59dc816a923c6c1d3