ensureSerialPermission

Waits up to waitMillis for a serial device to be present and ensures the app holds USB permission for it, prompting the user when needed.

Post-update verification depends on this: every USB re-enumeration is a new device identity to Android, so the grant earned before an update does not carry across the device's reboot — without a fresh one the transport's auto-recovery fails with SecurityException and verification times out on a healthy device.

Advisory: reconnection still rides the transport's own recovery; a denial just means verification times out as it would have anyway.