Address Android API Council feedback for Serial API.

In SerialManager.registerSerialPortListener:
 - Make the listener the last parameter in SerialManager.registerSerialPortListener for consistency and a more idiomatic usage in kotlin clients.
 - Add the throw tag for the IllegalStateException and the @CallbackExecutor annotation.

In SerialPort.requestOpen:
 - Introduce public constants for the flag and define an @IntDef for them.

Bug: 427450020
Bug: 427647872
Flag: android.hardware.serial.flags.enable_serial_api
Test: manual
Change-Id: I266fb182f02eefce5e05a78114f266fbe112dab5