Move billing code to shared module.

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

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util.billing
+
+import android.app.Activity
+
+/**
+ * Variant interface for the BillingApi.
+ */
+interface BillingApi {
+  fun isApiAvailable(): Boolean = false
+  suspend fun queryProducts() = Unit
+
+  /**
+   * Queries the user's current purchases. This enqueues a check and will
+   * propagate it to the normal callbacks in the api.
+   */
+  suspend fun queryPurchases() = Unit
+
+  suspend fun launchBillingFlow(activity: Activity) = Unit
+
+  /**
+   * Empty implementation, to be used when play services are available but
+   * GooglePlayBillingApi is not available.
+   */
+  object Empty : BillingApi
+}