Close executors in BasePcpHandler::Shutdown

The client can call DisconnectFromEndpointManager() explicitly before
BasePcpHandler is shut down. In this case, th executors were not terminated
inside BasePcpHandler::Shutdown()
This caused use-after-free errors in BasePcpHandlerTest.IoError_RequestConnectionFails

PiperOrigin-RevId: 551340708

connections/implementation/base_pcp_handler.cc

@@ -84,7 +84,7 @@ BasePcpHandler::~BasePcpHandler() {
 }
 
 void BasePcpHandler::Shutdown() {
-  if (stop_) return;
+  if (closed_.Set(true)) return;
   NEARBY_LOGS(INFO) << "Initiating shutdown of BasePcpHandler("
                     << strategy_.GetName() << ")";
   DisconnectFromEndpointManager();

connections/implementation/base_pcp_handler.h

@@ -609,6 +609,7 @@ class BasePcpHandler : public PcpHandler,
   Strategy strategy_{PcpToStrategy(pcp_)};
   EncryptionRunner encryption_runner_;
   BwuManager* bwu_manager_;
+  AtomicBoolean closed_{false};
 };
 
 }  // namespace connections