[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/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")
+}