[client] Support embed.Client on Android with netstack mode (#5623)

* [client] Support embed.Client on Android with netstack mode

embed.Client.Start() calls ConnectClient.Run() which passes an empty
MobileDependency{}. On Android, the engine dereferences nil fields
(IFaceDiscover, NetworkChangeListener, DnsReadyListener) causing panics.

Provide complete no-op stubs so the engine's existing Android code
paths work unchanged — zero modifications to engine.go:

- Add androidRunOverride hook in Run() for Android-specific dispatch
- Add runOnAndroidEmbed() with complete MobileDependency (all stubs)
- Wire default stubs via init() in connect_android_default.go:
  noopIFaceDiscover, noopNetworkChangeListener, noopDnsReadyListener
- Forward logPath to c.run()

Tested: embed.Client starts on Android arm64, joins mesh via relay,
discovers peers, localhost proxy works for TCP+UDP forwarding.

* [client] Fix TestServiceParamsPath for Windows path separators

Use filepath.Join in test assertions instead of hardcoded POSIX paths
so the test passes on Windows where filepath.Join uses backslashes.

client/internal/connect_android_embed.go

@@ -0,0 +1,32 @@
+//go:build android
+
+package internal
+
+import (
+	"net/netip"
+
+	"github.com/netbirdio/netbird/client/internal/dns"
+	"github.com/netbirdio/netbird/client/internal/listener"
+	"github.com/netbirdio/netbird/client/internal/stdnet"
+)
+
+// runOnAndroidEmbed is like RunOnAndroid but accepts a runningChan
+// so embed.Client.Start() can detect when the engine is ready.
+// It provides complete MobileDependency so the engine's existing
+// Android code paths work unchanged.
+func (c *ConnectClient) runOnAndroidEmbed(
+	iFaceDiscover stdnet.ExternalIFaceDiscover,
+	networkChangeListener listener.NetworkChangeListener,
+	dnsAddresses []netip.AddrPort,
+	dnsReadyListener dns.ReadyListener,
+	runningChan chan struct{},
+	logPath string,
+) error {
+	mobileDependency := MobileDependency{
+		IFaceDiscover:         iFaceDiscover,
+		NetworkChangeListener: networkChangeListener,
+		HostDNSAddresses:      dnsAddresses,
+		DnsReadyListener:      dnsReadyListener,
+	}
+	return c.run(mobileDependency, runningChan, logPath)
+}