Add autovalue to appcheck debug's javadoc classpath (#3524)

Author: rlazo

appcheck/firebase-appcheck-debug-testing/firebase-appcheck-debug-testing.gradle

@@ -52,6 +52,8 @@ dependencies {
     implementation 'com.google.android.gms:play-services-tasks:18.0.1'
     implementation 'androidx.test:core:1.2.0'
 
+    javadocClasspath 'com.google.auto.value:auto-value-annotations:1.6.6'
+
     testImplementation 'junit:junit:4.13-beta-2'
     testImplementation 'org.mockito:mockito-core:2.25.0'
     testImplementation "org.robolectric:robolectric:$robolectricVersion"