Minor cleanup

app/src/main/java/eu/kanade/tachiyomi/data/download/DownloadCache.kt

@@ -3,6 +3,7 @@ package eu.kanade.tachiyomi.data.download
 import android.content.Context
 import androidx.core.net.toUri
 import com.hippo.unifile.UniFile
+import eu.kanade.core.util.mapNotNullKeys
 import eu.kanade.domain.download.service.DownloadPreferences
 import eu.kanade.domain.manga.model.Manga
 import eu.kanade.tachiyomi.data.database.models.Chapter
@@ -68,9 +69,7 @@ class DownloadCache(
         downloadPreferences.downloadsDirectory().changes()
             .onEach {
                 rootDownloadsDir = RootDirectory(getDirectoryFromPreference())
-
-                // Invalidate cache
-                lastRenew = 0L
+                invalidateCache()
             }
             .launchIn(scope)
     }
@@ -225,6 +224,10 @@ class DownloadCache(
         notifyChanges()
     }
 
+    fun invalidateCache() {
+        lastRenew = 0L
+    }
+
     /**
      * Returns the downloads directory from the user's preferences.
      */
@@ -233,10 +236,6 @@ class DownloadCache(
         return UniFile.fromUri(context, dir.toUri())
     }
 
-    fun invalidateCache() {
-        lastRenew = 0L
-    }
-
     /**
      * Renews the downloads cache.
      */
@@ -315,15 +314,6 @@ class DownloadCache(
             _changes.send(Unit)
         }
     }
-
-    /**
-     * Returns a new map containing only the key entries of [transform] that are not null.
-     */
-    private inline fun <K, V, R> Map<out K, V>.mapNotNullKeys(transform: (Map.Entry<K?, V>) -> R?): ConcurrentHashMap<R, V> {
-        val mutableMap = ConcurrentHashMap<R, V>()
-        forEach { element -> transform(element)?.let { mutableMap[it] = element.value } }
-        return mutableMap
-    }
 }
 
 /**