Add copyApiTxtFile task (#6724)

Prerequisite of Metalava based SemVer

Author: emilypgoogle

plugins/src/main/java/com/google/firebase/gradle/plugins/CopyApiTask.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.firebase.gradle.plugins
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.file.RegularFileProperty
+import org.gradle.api.tasks.InputFile
+import org.gradle.api.tasks.OutputFile
+import org.gradle.api.tasks.TaskAction
+
+abstract class CopyApiTask : DefaultTask() {
+  @get:InputFile abstract val apiTxtFile: RegularFileProperty
+  @get:OutputFile abstract val output: RegularFileProperty
+
+  @TaskAction
+  fun run() {
+    output.get().asFile.writeText(apiTxtFile.get().asFile.readText())
+  }
+}

plugins/src/main/java/com/google/firebase/gradle/plugins/FirebaseAndroidLibraryPlugin.kt

@@ -160,6 +160,11 @@ class FirebaseAndroidLibraryPlugin : BaseFirebaseLibraryPlugin() {
           .getLatestReleasedVersion()
       )
     }
+
+    project.tasks.register<CopyApiTask>("copyApiTxtFile") {
+      apiTxtFile.set(project.file("api.txt"))
+      output.set(project.file("previous_api.txt"))
+    }
   }
 
   private fun setupApiInformationAnalysis(project: Project, android: LibraryExtension) {

plugins/src/main/java/com/google/firebase/gradle/plugins/FirebaseJavaLibraryPlugin.kt

@@ -103,6 +103,11 @@ class FirebaseJavaLibraryPlugin : BaseFirebaseLibraryPlugin() {
 
       dependsOn("copyPreviousArtifacts")
     }
+
+    project.tasks.register<CopyApiTask>("copyApiTxtFile") {
+      apiTxtFile.set(project.file("api.txt"))
+      output.set(project.file("previous_api.txt"))
+    }
   }
 
   private fun setupApiInformationAnalysis(project: Project) {

plugins/src/main/java/com/google/firebase/gradle/plugins/FirebaseLibraryExtension.kt

@@ -241,6 +241,9 @@ constructor(val project: Project, val type: LibraryType) {
   val version: String
     get() = project.version.toString()
 
+  val previousVersion: String
+    get() = project.properties["latestReleasedVersion"].toString()
+
   val path: String = project.path
 
   val runtimeClasspath: String =