DownloadWorker: Move downloads observing logic into worker as well

This allows us to keep all the modifications related to DownloadWorker in a
single place.

Also, drop the 3seconds delay and do the cleanup instead of delegating it to
worker if last job was failed. This is better and more error-proof.

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

app/src/main/java/com/aurora/extensions/Collection.kt

@@ -28,15 +28,3 @@ fun <T> MutableSet<T>.flushAndAdd(list: Set<T>) {
     clear()
     addAll(list)
 }
-
-fun <T> MutableSet<T>.copyAndAdd(element: T): MutableSet<T> {
-    val newSet = this.toMutableSet()
-    newSet.add(element)
-    return newSet
-}
-
-fun <T> MutableSet<T>.copyAndRemove(element: T): MutableSet<T> {
-    val newSet = this.toMutableSet()
-    newSet.remove(element)
-    return newSet
-}