Address comments #1

Author: diwu-arete

firebase-segmentation/src/main/java/com/google/firebase/segmentation/FirebaseSegmentation.java

@@ -79,7 +79,8 @@ public static FirebaseSegmentation getInstance(@NonNull FirebaseApp app) {
     return app.get(FirebaseSegmentation.class);
   }
 
-  Task<Void> setCustomInstallationId(@Nullable String customInstallationId) {
+  @NonNull
+  public synchronized Task<Void> setCustomInstallationId(@Nullable String customInstallationId) {
     if (customInstallationId == null) {
       return clearCustomInstallationId();
     }

firebase-segmentation/src/main/java/com/google/firebase/segmentation/local/CustomInstallationIdCache.java

@@ -101,13 +101,7 @@ private String getSharedPreferencesKey(String key) {
 
   private Task<Boolean> commitSharedPreferencesEditAsync(SharedPreferences.Editor editor) {
     TaskCompletionSource<Boolean> result = new TaskCompletionSource<>();
-    ioExecuter.execute(
-        new Runnable() {
-          @Override
-          public void run() {
-            result.setResult(editor.commit());
-          }
-        });
+    ioExecuter.execute(() -> result.setResult(editor.commit()));
     return result.getTask();
   }
 }