playback: show notification progress for unknown-length streams

Media3 marks a progressive stream live when the data source reports no
length and the extractor no duration, and a live item makes the media
session publish PLAYBACK_POSITION_UNKNOWN, so the notification progress
never advances even though the item carries the library duration.

The app only declares a stream length for a transcode of a track that has
one in a length-estimable format (`declaredStreamLength`), so a chunked
response stays live for an Opus/Ogg/Vorbis transcode (a length estimate
breaks the end-seek they rely on), for a transcode of a track whose
library duration is missing, and for a direct play the server does not
give a Content-Length. The now playing screen shows progress in all of
those because it falls back to the metadata duration; the notification
had nothing to advance.

Wrap the ExoPlayer in a ForwardingPlayer that reports an item with a
declared duration as finite. The session then publishes the real position
and the metadata duration, so the notification shows progress for every
unknown-length stream, not just Opus. The stream is still not seekable;
only the reported liveness changes.

app/src/main/kotlin/com/subtracks/playback/KnownDurationPlayer.kt

@@ -0,0 +1,30 @@
+package com.subtracks.playback
+
+import androidx.annotation.OptIn
+import androidx.media3.common.ForwardingPlayer
+import androidx.media3.common.Player
+import androidx.media3.common.util.UnstableApi
+
+/**
+ * Reports a track whose duration is known from its metadata as finite rather than live.
+ *
+ * Media3 marks a progressive stream live when the data source reports no length and the extractor
+ * no duration. The app declares a length only for a transcode of a track that has one in a
+ * length-estimable format, so a chunked response stays live for an Opus/Ogg/Vorbis transcode, for
+ * a transcode of a track whose library duration is missing, and for a direct play the server does
+ * not give a Content-Length. A live item makes the media session publish no playback position, so
+ * the notification loses its progress even though the item carries a real duration. The stream is
+ * still not seekable; this only restores the position the session reports.
+ */
+@OptIn(UnstableApi::class)
+internal class KnownDurationPlayer(
+    player: Player,
+) : ForwardingPlayer(player) {
+    override fun isCurrentMediaItemLive(): Boolean = if (declaredDurationMs() != null) false else super.isCurrentMediaItemLive()
+
+    private fun declaredDurationMs(): Long? =
+        currentMediaItem
+            ?.mediaMetadata
+            ?.durationMs
+            ?.takeIf { it > 0 }
+}

app/src/main/kotlin/com/subtracks/playback/PlaybackService.kt

@@ -130,6 +130,7 @@ class PlaybackService : MediaSessionService() {
                 }
             },
         )
+        val player = KnownDurationPlayer(exoPlayer)
         val loader =
             CoverArtBitmapLoader(
                 applicationContext,
@@ -139,7 +140,7 @@ class PlaybackService : MediaSessionService() {
         artworkLoader = loader
         session =
             MediaSession
-                .Builder(this, exoPlayer)
+                .Builder(this, player)
                 .setSessionActivity(
                     PendingIntent.getActivity(
                         this,