use a default list constructor on mediator

Author: polbins

core/src/main/java/com/amplitude/core/platform/Mediator.kt

@@ -6,7 +6,9 @@ import com.amplitude.core.events.IdentifyEvent
 import com.amplitude.core.events.RevenueEvent
 import java.util.concurrent.CopyOnWriteArrayList
 
-internal class Mediator(private val plugins: CopyOnWriteArrayList<Plugin>) {
+internal class Mediator(
+    private val plugins: CopyOnWriteArrayList<Plugin> = CopyOnWriteArrayList()
+) {
     fun add(plugin: Plugin) {
         plugins.add(plugin)
     }

core/src/main/java/com/amplitude/core/platform/Timeline.kt

@@ -2,14 +2,13 @@ package com.amplitude.core.platform
 
 import com.amplitude.core.Amplitude
 import com.amplitude.core.events.BaseEvent
-import java.util.concurrent.CopyOnWriteArrayList
 
 open class Timeline {
     internal val plugins: Map<Plugin.Type, Mediator> = mapOf(
-        Plugin.Type.Before to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Enrichment to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Destination to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Utility to Mediator(CopyOnWriteArrayList())
+        Plugin.Type.Before to Mediator(),
+        Plugin.Type.Enrichment to Mediator(),
+        Plugin.Type.Destination to Mediator(),
+        Plugin.Type.Utility to Mediator()
     )
     lateinit var amplitude: Amplitude