[client] Implement environment variable handling for Android (#4440)

Some features can only be manipulated via environment variables. With this PR, environment variables can be managed from Android.

client/internal/peer/conn.go

@@ -6,7 +6,6 @@ import (
 	"math/rand"
 	"net"
 	"net/netip"
-	"os"
 	"runtime"
 	"sync"
 	"time"
@@ -174,7 +173,7 @@ func (conn *Conn) Open(engineCtx context.Context) error {
 	conn.handshaker = NewHandshaker(conn.Log, conn.config, conn.signaler, conn.workerICE, conn.workerRelay)
 
 	conn.handshaker.AddOnNewOfferListener(conn.workerRelay.OnNewOffer)
-	if os.Getenv("NB_FORCE_RELAY") != "true" {
+	if !isForceRelayed() {
 		conn.handshaker.AddOnNewOfferListener(conn.workerICE.OnNewOffer)
 	}
 

client/internal/peer/env.go

@@ -0,0 +1,14 @@
+package peer
+
+import (
+	"os"
+	"strings"
+)
+
+const (
+	EnvKeyNBForceRelay = "NB_FORCE_RELAY"
+)
+
+func isForceRelayed() bool {
+	return strings.EqualFold(os.Getenv(EnvKeyNBForceRelay), "true")
+}