Perform download cache renewal async

Don't block on cache renewals, but notify library on updates so that the badges show up when ready.

We skip the cache when checking if a chapter is downloaded for the reader assuming that it's a
relatively low cost to check for a single chapter.

(Probably) fixes #8254 / fixes #7847

app/src/main/java/eu/kanade/tachiyomi/extension/ExtensionManager.kt

@@ -42,6 +42,9 @@ class ExtensionManager(
     private val preferences: SourcePreferences = Injekt.get(),
 ) {
 
+    var isInitialized = false
+        private set
+
     /**
      * API where all the available extensions can be found.
      */
@@ -102,6 +105,8 @@ class ExtensionManager(
         _untrustedExtensionsFlow.value = extensions
             .filterIsInstance<LoadResult.Untrusted>()
             .map { it.extension }
+
+        isInitialized = true
     }
 
     /**