binder: Suppress GuardedBy("this") warning for test access (#12362)

Co-authored-by: MV Shiva <[email protected]>

Author: kannanjgithub

binder/src/main/java/io/grpc/binder/internal/BinderTransport.java

@@ -574,6 +574,7 @@ Map<Integer, Inbound<?>> getOngoingCalls() {
   }
 
   @VisibleForTesting
+  @SuppressWarnings("GuardedBy")
   LeakSafeOneWayBinder getIncomingBinderForTesting() {
     return this.incomingBinder;
   }