cmd/tailscale: enabled logtail flushing for the Android client

The c2n endpoint was not working since we did not configure the flush
function (same issue that was previously fixed in the iOS client in
tailscale/corp#9939).

Updates tailscale/corp#8564

cmd/tailscale/backend.go

@@ -45,6 +45,7 @@ type backend struct {
 	lastDNSCfg *dns.OSConfig
 
 	logIDPublic string
+	logger      *logtail.Logger
 
 	// avoidEmptyDNS controls whether to use fallback nameservers
 	// when no nameservers are provided by Tailscale.
@@ -152,6 +153,9 @@ func newBackend(dataDir string, jvm *jni.JVM, appCtx jni.Object, store *stateSto
 	if err := ns.Start(lb); err != nil {
 		return nil, fmt.Errorf("startNetstack: %w", err)
 	}
+	if b.logger != nil {
+		lb.SetLogFlusher(b.logger.StartFlush)
+	}
 	b.engine = engine
 	b.backend = lb
 	return b, nil
@@ -343,10 +347,10 @@ func (b *backend) SetupLogs(logDir string, logID logid.PrivateID) {
 	}
 
 	logf := logger.RusagePrefixLog(log.Printf)
-	tlog := logtail.NewLogger(logcfg, logf)
+	b.logger = logtail.NewLogger(logcfg, logf)
 
 	log.SetFlags(0)
-	log.SetOutput(tlog)
+	log.SetOutput(b.logger)
 
 	log.Printf("goSetupLogs: success")