to save internal RAM space

This increases CPU usage by ~2% for the audio_conv task, which I
think is an acceptable tradeoff. The Speex resampler still
makes its own allocations in internal RAM.

The SetTrack event is sent when *decoding* finishes, not when
*playback* finishes so the decoder can start buffering the next
track in advance of its playback to ensure gapless playback.
Clearing StreamCues causes
Playback::react(const internal::StreamHeartbeat& ev)
to transit the Playback FSM to Standby, which sets the
IAudioOutput to stop reading from the output buffer. Thus,
clearing StreamCues in response to the SetTrack event paused
playback with the end of the track stuck waiting in the output
buffer. When a new queue was loaded and started playing, the
end of the previous track would be played before the new track
was played.

Fixes https://codeberg.org/cool-tech-zone/tangara-fw/issues/313