Upcast ConcurrentHashMap to Map to avoid compatibility issue

Resovles ReactiveX#4653

See http://stackoverflow.com/a/32955708/61158

Author: ZacSweers

src/main/java/io/reactivex/internal/schedulers/SchedulerPoolFactory.java

@@ -16,13 +16,15 @@
 
 package io.reactivex.internal.schedulers;
 
-import java.util.*;
-import java.util.concurrent.*;
-import java.util.concurrent.atomic.AtomicReference;
-
 import io.reactivex.internal.util.SuppressAnimalSniffer;
 import io.reactivex.plugins.RxJavaPlugins;
 
+import java.util.ArrayList;
+import java.util.Map;
+import java.util.Properties;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicReference;
+
 /**
  * Manages the creating of ScheduledExecutorServices and sets up purging.
  */
@@ -46,7 +48,9 @@ public enum SchedulerPoolFactory {
     static final AtomicReference<ScheduledExecutorService> PURGE_THREAD =
             new AtomicReference<ScheduledExecutorService>();
 
-    static final ConcurrentHashMap<ScheduledThreadPoolExecutor, Object> POOLS =
+    // Upcast to the Map interface here to avoid 8.x compatibility issues.
+    // See http://stackoverflow.com/a/32955708/61158
+    static final Map<ScheduledThreadPoolExecutor, Object> POOLS =
             new ConcurrentHashMap<ScheduledThreadPoolExecutor, Object>();
 
     /**