Android birthday field does not retain `/` date separators when manually editing

Can reproduce CR: this field needs more restrictive inputs. Unexpected Keyboard can type slashes while in the number mode, but this field should also stop extra slashes from being added. Version 2026.09.100, Beta 2, Channel Beta, Build 2527, Android 16 (arm64), Device Google Pixel 9a, Locale en-US, Push fcm, Flutter 3.47.5, Dart 3.13.4