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

app/src/main/java/eu/kanade/tachiyomi/ui/download/DownloadQueueScreenModel.kt

@@ -14,7 +14,6 @@ import kotlinx.coroutines.Job
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.asStateFlow
-import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.debounce
@@ -22,8 +21,6 @@ import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
-import logcat.LogPriority
-import tachiyomi.core.util.system.logcat
 import uy.kohesive.injekt.Injekt
 import uy.kohesive.injekt.api.get
 
@@ -116,8 +113,7 @@ class DownloadQueueScreenModel(
 
     init {
         coroutineScope.launch {
-            downloadManager.queue.updates
-                .catch { logcat(LogPriority.ERROR, it) }
+            downloadManager.queue.state
                 .map { downloads ->
                     downloads
                         .groupBy { it.source }

app/src/main/java/eu/kanade/tachiyomi/ui/more/MoreTab.kt

@@ -95,14 +95,13 @@ private class MoreScreenModel(
         coroutineScope.launchIO {
             combine(
                 DownloadService.isRunning,
-                downloadManager.queue.updates,
+                downloadManager.queue.state,
             ) { isRunning, downloadQueue -> Pair(isRunning, downloadQueue.size) }
                 .collectLatest { (isDownloading, downloadQueueSize) ->
                     val pendingDownloadExists = downloadQueueSize != 0
                     _state.value = when {
                         !pendingDownloadExists -> DownloadQueueState.Stopped
-                        !isDownloading && !pendingDownloadExists -> DownloadQueueState.Paused(0)
-                        !isDownloading && pendingDownloadExists -> DownloadQueueState.Paused(downloadQueueSize)
+                        !isDownloading -> DownloadQueueState.Paused(downloadQueueSize)
                         else -> DownloadQueueState.Downloading(downloadQueueSize)
                     }
                 }