audio: Allow specifying "default" stream type in V7

Despite that AUDIO_STREAM_DEFAULT value of audio_stream_type_t
should only used by the framework, it can still end up being
passed to the HAL in port configs and offload info structures.
This happens in the cases when the stream type is not actually
used by the HAL. It seems natural to use an empty string
as the value of AudioStreamType field in this case.

Bug: 179743630
Test: atest android.hardware.audio.common@7.0-util_tests
Test: make a telephone call on a device with HAL V7
Change-Id: Ia330031fca9d081627746b4f6074162835c4c54b