fix(playback): hoist DataSourceBitmapLoader build into a function

Android Lint's UnsafeOptInUsageError doesn't recognize @OptIn placed on a
`by lazy` property as covering the lambda body, so each Media3 unstable-API
call inside the initializer (Builder, setExecutorService, setDataSourceFactory,
build) was flagged. Move the construction into a real function carrying the
@OptIn annotation; the lazy delegate just calls it. No behavior change.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/playerPool/MediaSessionPool.kt

@@ -81,14 +81,17 @@ class MediaSessionPool(
     // The bitmap loader is stateless w.r.t. the session; a fresh allocation per session was
     // pure noise. ExoPlayer's DEFAULT_EXECUTOR_SERVICE is a process-wide singleton, the
     // dataSourceFactory is owned by the pool, and the appContext is already retained.
+    // The init is in a separate function so the @OptIn lands on a real declaration —
+    // applying it to a `by lazy` property doesn't propagate into the lambda body.
+    private val sharedBitmapLoader by lazy { buildSharedBitmapLoader() }
+
     @OptIn(UnstableApi::class)
-    private val sharedBitmapLoader by lazy {
+    private fun buildSharedBitmapLoader(): DataSourceBitmapLoader =
         DataSourceBitmapLoader
             .Builder(appContext)
             .setExecutorService(DataSourceBitmapLoader.DEFAULT_EXECUTOR_SERVICE.get())
             .setDataSourceFactory(dataSourceFactory)
             .build()
-    }
 
     // protects from LruCache killing playing sessions
     private val playingMap = mutableMapOf<String, SessionListener>()