Piano: Use AudioServer instead of /dev/audio for audio

Piano is an old application that predates AudioServer. For this reason,
it was architected to directly talk to the soundcard via the /dev/audio
device. This caused multiple problems including simultaneous playback
issues, no ability to change volume/mute for Piano and more.

This change moves Piano to use AudioServer like any well-behaved audio
application :^) The track processing and IPC communication is moved to
the main thread because IPC doesn't like multi-threading. For this, the
new AudioPlayerLoop class is utilized that should evolve into the
DSP->AudioServer interface in the future.

Because Piano's CPU utilization has gotten so low (about 3-6%), the UI
update loop is switched back to render at exactly 60fps.

This is an important commit on the road to #6528.

Userland/Applications/Piano/Music.h

@@ -23,7 +23,7 @@ struct Sample {
     i16 right;
 };
 
-constexpr int sample_count = 1024;
+constexpr int sample_count = 1 << 9;
 
 constexpr int buffer_size = sample_count * sizeof(Sample);