[client] Fix engine restart (#3435)

- Refactor the network monitoring to handle one event and it after return
- In the engine restart cancel the upper layer context and the responsibility of the engine stop will be the upper layer
- Before triggering a restart, the engine checks whether the state is already down. This helps avoid unnecessary delayed network restart events.

client/internal/networkmonitor/monitor_mobile.go

@@ -2,10 +2,21 @@
 
 package networkmonitor
 
-import "context"
+import (
+	"context"
+	"fmt"
+)
 
-func (nw *NetworkMonitor) Start(context.Context, func()) error {
-	return nil
+type NetworkMonitor struct {
+}
+
+// New creates a new network monitor.
+func New() *NetworkMonitor {
+	return &NetworkMonitor{}
+}
+
+func (nw *NetworkMonitor) Listen(_ context.Context) error {
+	return fmt.Errorf("network monitor not supported on mobile platforms")
 }
 
 func (nw *NetworkMonitor) Stop() {