binder: Fix synchronization instead of suppressing GuardedBy

There's no reason why we shouldn't just have proper synchronization
here, even if only used in tests. We shouldn't get into the habit of
suppressing them.

Author: ejona86

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

@@ -574,8 +574,7 @@ Map<Integer, Inbound<?>> getOngoingCalls() {
   }
 
   @VisibleForTesting
-  @SuppressWarnings("GuardedBy")
-  LeakSafeOneWayBinder getIncomingBinderForTesting() {
+  synchronized LeakSafeOneWayBinder getIncomingBinderForTesting() {
     return this.incomingBinder;
   }