fix(android): look gains up by key, not value (M464 #5000)

`id in map` on a ConcurrentHashMap resolves to its legacy contains(),
which tests values (KT-18053); the compiler refuses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/player/gain/ReplayGainStore.kt

@@ -64,7 +64,7 @@ class ReplayGainStore internal constructor(
     fun request(trackIds: Collection<String>) {
         val now = clock()
         val wanted = trackIds.filter { id ->
-            id !in gains &&
+            !gains.containsKey(id) &&
                 (missedAt[id]?.let { now - it > MISS_RETRY_MS } ?: true) &&
                 inflight.add(id)
         }
@@ -80,7 +80,7 @@ class ReplayGainStore internal constructor(
 
     internal suspend fun load(ids: List<String>) {
         fromCache(ids)
-        val rest = ids.filter { it !in gains }
+        val rest = ids.filter { !gains.containsKey(it) }
         val health = serverHealth()
         if (rest.isEmpty() || health == ServerHealth.Offline || health == ServerHealth.ServerDown) return
         for (batch in rest.chunked(MAX_IDS_PER_REQUEST)) {