add a song info dialog and show the encoding in the manager

The song context menus gain Info, which lists every field we hold for the song
and, when it is downloaded, the file's format, bitrate, sample rate, channels
and size read from the file itself. For the song currently playing it takes the
encoding from the media3 stream instead, so a transcoded stream reports what is
actually being played. The menu is shared, so this reaches the album, playlist,
now playing and queue menus at once.

The downloads manager shows the same format and bitrate beside the size on each
song row, read lazily for the rows on screen rather than for the whole tree.

app/src/test/kotlin/com/subtracks/data/model/AudioEncodingTest.kt

@@ -0,0 +1,25 @@
+package com.subtracks.data.model
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class AudioEncodingTest {
+    @Test
+    fun commonTypesGetReadableNames() {
+        assertEquals("MP3", AudioEncoding("audio/mpeg").format)
+        assertEquals("FLAC", AudioEncoding("audio/flac").format)
+        assertEquals("AAC", AudioEncoding("audio/mp4").format)
+        assertEquals("Opus", AudioEncoding("audio/opus").format)
+    }
+
+    @Test
+    fun anUnknownTypeFallsBackToItsSubtype() {
+        assertEquals("X-CUSTOM", AudioEncoding("audio/x-custom").format)
+    }
+
+    @Test
+    fun noTypeMeansNoFormat() {
+        assertNull(AudioEncoding(null).format)
+    }
+}

app/src/test/kotlin/com/subtracks/ui/downloads/DownloadTreeTest.kt

@@ -54,6 +54,7 @@ class DownloadTreeTest {
         size: Long,
     ) = DownloadedSong(
         songId = id,
+        sourceId = 1,
         title = title,
         albumId = albumId,
         albumName = albumName,