music: add disc number indexing

Change the indexer to parse out the disc number field from
AudioColumns.TRACK.

On newer versions, I hope to further modify the loader to use
the API 30+ CD_TRACK_NUMBER field, as it is more versatile than
TRACK.

app/src/main/java/org/oxycblt/auxio/detail/DetailViewModel.kt

@@ -139,6 +139,7 @@ class DetailViewModel : ViewModel() {
         val data = mutableListOf<Item>(album)
         data.add(SortHeader(id = -2, R.string.lbl_songs))
         data.add(DiscHeader(id = -3, 1))
+        data.addAll(albumSort.album(album))
         _albumData.value = data
     }
 }

app/src/main/java/org/oxycblt/auxio/music/Indexer.kt

@@ -192,8 +192,18 @@ object Indexer {
                     // and T is the track. This is dumb and insane and forces me to mangle track
                     // numbers above 1000, but there is nothing we can do that won't break the app
                     // below API 30.
-                    // TODO: Disk number support?
+                    var track: Int? = null
-                    val track = cursor.getIntOrNull(trackIndex)?.mod(1000)
+                    var disc: Int? = null
+
+                    val rawTrack = cursor.getIntOrNull(trackIndex)
+                    if (rawTrack != null) {
+                        track = rawTrack % 1000
+                        disc = rawTrack / 1000
+                        if (disc == 0) {
+                            // A disc number of 0 means that there is no disc.
+                            disc = null
+                        }
+                    }
 
                     val duration = cursor.getLong(durationIndex)
                     val year = cursor.getIntOrNull(yearIndex)
@@ -225,6 +235,7 @@ object Indexer {
                             fileName,
                             duration,
                             track,
+                            disc,
                             id,
                             year,
                             album,

app/src/main/java/org/oxycblt/auxio/music/Music.kt

@@ -68,6 +68,8 @@ data class Song(
     val duration: Long,
     /** The track number of this song, null if there isn't any. */
     val track: Int?,
+    /** The disc number of this song, null if there isn't any. */
+    val disc: Int?,
     /** Internal field. Do not use. */
     val _mediaStoreId: Long,
     /** Internal field. Do not use. */