Add gRPC's AbstractStub to WellKnownThreadSafety for EP and AL

PiperOrigin-RevId: 915431942

Author: java-team-github-bot

core/src/main/java/com/google/errorprone/bugpatterns/threadsafety/WellKnownThreadSafety.java

@@ -131,6 +131,7 @@ private static ImmutableMap<String, AnnotationInfo> buildThreadSafeClasses(
         .add("io.reactivex.Single", "T")
         .add("io.reactivex.Flowable", "T")
         .add(Throwable.class) // Unsafe due to initCause, but generally used across threads
+        .add("io.grpc.stub.AbstractStub")
         .add("java.lang.ThreadLocal")
         .add("java.lang.invoke.MethodHandle")
         .add(java.lang.reflect.Method.class)