Add logic for MODE_ASSISTANT_CONVERSATION mode

The hardware button keys will be forwarded to STREAM_ASSISTANT. In
absolute volume mode STREAM_ASSISTANT will drive the absolute volume

Test: manual
Bug: 416329698
Flag: android.media.audio.assistant_volume_control

Change-Id: Ic8f9d8bab6fcc56d2247b77e62a19244fdfe7180