Replace RxJava in DownloadQueue (#9016)

* Misc cleanup

- Replace !List.isEmpty with List.isNotEmpty
- Remove redundant case in MoreScreenModel
- Drop no-op StateFlow.catch
  - From lint warning:
> SharedFlow never completes, so this operator typically has not
> effect, it can only catch exceptions from 'onSubscribe' operator

* Convert DownloadQueue queue to MutableStateFlow

Replace delegation to a MutableList with an internal
MutableStateFlow<List>.

In order to avoid modifying every usage of the queue as a list, add
passthrough functions for the currently used list functions. This
should be later refactored, possibly by inlining DownloadQueue
into Downloader.

DownloadQueue.updates was a SharedFlow which updated every time a
change was made to the queue. This is now equivalent to the queue
StateFlow.

Simultaneous assignments to _state.value could cause concurrency
issues. To avoid this, always modify the queue using _state.update.

* Add Download.statusFlow/progressFlow

progressFlow is based on the DownloadQueueScreenModel implementation
rather than the DownloadQueue implementation.

* Reimplement DownloadQueue.statusFlow/progressFlow

Use StateFlow<List<T>>.flatMapLatest() and List<Flow<T>>.merge() to
replicate the effect of PublishSubject.

Use drop(1) to avoid re-emitting the state of each download each time
the merged flow is recreated.

* fixup! Reimplement DownloadQueue.statusFlow/progressFlow

source-api/src/main/java/eu/kanade/tachiyomi/source/model/Page.kt

@@ -6,7 +6,6 @@ import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.Transient
-import rx.subjects.Subject
 
 @Serializable
 open class Page(
@@ -28,7 +27,6 @@ open class Page(
         get() = _statusFlow.value
         set(value) {
             _statusFlow.value = value
-            statusSubject?.onNext(value)
         }
 
     @Transient
@@ -42,9 +40,6 @@ open class Page(
             _progressFlow.value = value
         }
 
-    @Transient
-    var statusSubject: Subject<State, State>? = null
-
     override fun update(bytesRead: Long, contentLength: Long, done: Boolean) {
         progress = if (contentLength > 0) {
             (100 * bytesRead / contentLength).toInt()