The Storage card has been showing "0 B" for users who only stream
(never explicitly pin or download an album). usageBytes() summed
SUM(size_bytes) from audio_cache_index — but the streaming path
through audio_handler writes files via LockCachingAudioSource without
ever inserting an index row, so the index undercounts (often to
zero) for normal use.
Walk the cache directory instead. Catches everything on disk:
manually pinned tracks (registered in the index), stream-cached
tracks (LockCaching), partial downloads. Falls back gracefully when
a file is racing against concurrent writes / deletes.
Eviction still operates on the index (it needs the source/recency
metadata to pick eviction order). Stream-cached files aren't subject
to eviction today — separate problem; addressed when we wire a
download-complete hook from LockCaching back into the index.
bvandeusen