Fix stored stub sources not loading and being rewritten on every reload

Stub sources loaded from the database were written into a throwaway copy of the map, so they never reached the source manager. StubSource also becomes a data class, so a stored one compares equal to the same source again and isn't rewritten on every reload.

app/src/main/java/eu/kanade/tachiyomi/source/AndroidSourceManager.kt

@@ -66,9 +66,8 @@ class AndroidSourceManager(
         scope.launch {
             sourceRepository.subscribeAll()
                 .collectLatest { sources ->
-                    val mutableMap = stubSourcesMap.toMutableMap()
                     sources.forEach {
-                        mutableMap[it.id] = it
+                        stubSourcesMap[it.id] = it
                     }
                 }
         }

domain/src/main/java/tachiyomi/domain/source/model/StubSource.kt

@@ -8,7 +8,7 @@ import eu.kanade.tachiyomi.source.model.SChapter
 import eu.kanade.tachiyomi.source.model.SManga
 import eu.kanade.tachiyomi.source.model.SMangaUpdate
 
-class StubSource(
+data class StubSource(
     override val id: Long,
     override val lang: String,
     override val name: String,