Start re-work of play billing checkout flow.

core-util/src/main/java/org/signal/core/util/billing/BillingApi.kt

@@ -6,13 +6,22 @@
 package org.signal.core.util.billing
 
 import android.app.Activity
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.emptyFlow
 
 /**
  * Variant interface for the BillingApi.
  */
 interface BillingApi {
+  /**
+   * Listenable stream of billing purchase results. It's up to the user
+   * to call queryPurchases after subscription.
+   */
+  fun getBillingPurchaseResults(): Flow<BillingPurchaseResult> = emptyFlow()
+
   fun isApiAvailable(): Boolean = false
-  suspend fun queryProducts() = Unit
+
+  suspend fun queryProduct(): BillingProduct? = null
 
   /**
    * Queries the user's current purchases. This enqueues a check and will

core-util/src/main/java/org/signal/core/util/billing/BillingProduct.kt

@@ -0,0 +1,15 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util.billing
+
+import org.signal.core.util.money.FiatMoney
+
+/**
+ * Represents a purchasable product from the Google Play Billing API
+ */
+data class BillingProduct(
+  val price: FiatMoney
+)

core-util/src/main/java/org/signal/core/util/billing/BillingPurchaseResult.kt

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util.billing
+
+/**
+ * Sealed class hierarchy representing the different success
+ * and error states of google play billing purchases.
+ */
+sealed interface BillingPurchaseResult {
+  data class Success(
+    val purchaseToken: String,
+    val isAcknowledged: Boolean,
+    val purchaseTime: Long
+  ) : BillingPurchaseResult
+  data object UserCancelled : BillingPurchaseResult
+  data object None : BillingPurchaseResult
+  data object TryAgainLater : BillingPurchaseResult
+  data object AlreadySubscribed : BillingPurchaseResult
+  data object FeatureNotSupported : BillingPurchaseResult
+  data object GenericError : BillingPurchaseResult
+  data object NetworkError : BillingPurchaseResult
+  data object BillingUnavailable : BillingPurchaseResult
+}