Chnage visibility of EventMetadata

Author: tejasd

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/metadata/EventMetadata.kt

@@ -22,7 +22,7 @@ package com.google.firebase.crashlytics.internal.metadata
  * @property additionalCustomKeys a [Map<String, String>] of key value pairs to attach to the event,
  * in addition to the global custom keys.
  */
-data class EventMetadata
+internal data class EventMetadata
 @JvmOverloads
 constructor(
   val sessionId: String,