HfpClientInterface: fix decoder client suspend

|Decode::CancelStreamingRequest| should propagate the event to the
decoder interface, rather than to the encoder interface.

Bug: 349290628
Bug: 315234036
Flag: com::android::bluetooth::flags::is_sco_managed_by_audio and HFP
Test: atest --host bluetooth-test-audio-hal-hfp-client-interface
Change-Id: Ib644c143ca88437519371e9a7cb3a03685b64d7b