Use CLASS retention for @autoservice

Gradle incremental aggregating processors require all processed annotations to have CLASS retention so that the incremental compiler can substitute a class file for a source file if the file is unchanged. With SOURCE retention, Gradle requires a full rebuild.

See:
#615
https://docs.gradle.org/6.0.1/userguide/java_plugin.html#aggregating_annotation_processors
https://github.com/gradle/gradle/blob/v6.0.1/subprojects/language-java/src/main/java/org/gradle/api/internal/tasks/compile/processing/AggregatingProcessingStrategy.java#L49-L56

RELNOTES=`AutoService`: Use `CLASS` retention to support Gradle aggregating incremental annotation processors

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=282380619

service/annotations/src/main/java/com/google/auto/service/AutoService.java

@@ -16,7 +16,7 @@
 package com.google.auto.service;
 
 import static java.lang.annotation.ElementType.TYPE;
-import static java.lang.annotation.RetentionPolicy.SOURCE;
+import static java.lang.annotation.RetentionPolicy.CLASS;
 
 import java.lang.annotation.Documented;
 import java.lang.annotation.Retention;
@@ -36,7 +36,7 @@
  * </ul>
  */
 @Documented
-@Retention(SOURCE)
+@Retention(CLASS)
 @Target(TYPE)
 public @interface AutoService {
   /** Returns the interfaces implemented by this service provider. */