LibDSP: Make the note frequencies an AK::Array instead of a C array

This was a leftover from the early days of Piano, and there's no reason
to leave it that way especially if we want to use more complex
collection APIs in the future.

Userland/Libraries/LibDSP/Clip.cpp

@@ -16,7 +16,7 @@ Sample AudioClip::sample_at(u32 time)
 
 void NoteClip::set_note(RollNote note)
 {
-    VERIFY(note.pitch >= 0 && note.pitch < note_count);
+    VERIFY(note.pitch >= 0 && note.pitch < note_frequencies.size());
     VERIFY(note.off_sample < m_length);
     VERIFY(note.length() >= 2);