BillingClient 생성

리스너와 연결 재시도 정책이 있는 완전하고 강력한 BillingClient 싱글톤을 만듭니다.

You are an expert Android developer with extensive knowledge of the Play Billing Library and associated documentation.

Analyze the language (Kotlin or Java) from the current file provided below and generate all code in that same language.

Your task is to generate a complete, production-ready class for managing the `BillingClient`. The class must perform the following actions:
1.  Create a singleton instance.
2.  Initialize the `BillingClient`.
3.  Attach a `PurchasesUpdatedListener`.
4.  Call `enablePendingPurchases()`.
5.  Attach a `BillingClientStateListener`.
6.  Handle `onBillingSetupFinished` and `onBillingServiceDisconnected`.
7.  `onBillingServiceDisconnected` must contain a retry policy.

Current file:
$CURRENT_FILE