android-interop-testing: Workaround build mergeExtDexDebug race

4de4718 upgraded android-interop-testing to SDK version 23, but this
had previously been avoided because it triggered a Gradle or AGP bug.
The race happened to not trigger locally or for the PR's CI and the
change was merged. But the problem still was present, and the CI is
failing to build very frequently.

This works around the problem by explicitly adding a dependency from
mergeExtDexDebug. I didn't see any other mergeExtDex tasks created,
in particular mergeExtDexRelease. Hopefully we can remove this after
upgrading AGP or Gradle, but at least we can move forward with newer
Android API levels again.

Author: ejona86

android-interop-testing/build.gradle

@@ -111,6 +111,25 @@ tasks.withType(JavaCompile).configureEach {
             "|")
 }
 
+// Workaround error seen with Gradle 8.14.3 and AGP 7.4.1 when building:
+//   ./gradlew clean :grpc-android-interop-testing:build -PskipAndroid=false \
+//       -Pandroid.useAndroidX=true --no-build-cache
+//
+// Error message:
+//
+// Execution failed for task ':grpc-android-interop-testing:mergeExtDexDebug'.
+// > Could not resolve all files for configuration ':grpc-android-interop-testing:debugRuntimeClasspath'.
+//    > Failed to transform opencensus-contrib-grpc-metrics-0.31.1.jar (io.opencensus:opencensus-contrib-grpc-metrics:0.31.1) to match attributes {artifactType=android-dex, asm-transformed-variant=NONE, dexing-enable-desugaring=true, dexing-enable-jacoco-instrumentation=false, dexing-is-debuggable=true, dexing-min-sdk=23, org.gradle.category=library, org.gradle.libraryelements=jar, org.gradle.status=release, org.gradle.usage=java-runtime}.
+//       > Could not resolve all files for configuration ':grpc-android-interop-testing:debugRuntimeClasspath'.
+//          > Failed to transform grpc-api-1.81.0-SNAPSHOT.jar (project :grpc-api) to match attributes {artifactType=android-classes-jar, org.gradle.category=library, org.gradle.dependency.bundling=external, org.gradle.jvm.version=8, org.gradle.libraryelements=jar, org.gradle.usage=java-runtime}.
+//             > Execution failed for IdentityTransform: grpc-java/api/build/libs/grpc-api-1.81.0-SNAPSHOT.jar.
+//                > File/directory does not exist: grpc-java/api/build/libs/grpc-api-1.81.0-SNAPSHOT.jar
+tasks.configureEach { task ->
+    if (task.name.equals("mergeExtDexDebug")) {
+        dependsOn(':grpc-api:jar')
+    }
+}
+
 afterEvaluate {
     // Hack to workaround "Task ':grpc-android-interop-testing:extractIncludeDebugProto' uses this
     // output of task ':grpc-context:jar' without declaring an explicit or implicit dependency." The