Correct the serial flag's namespace

We also need to rename this flag according to go/aconfig-namespace.

NO_EXPORTED_FLAG_DELETION_CHECK=This feature isn't completed so the flag
is never enabled.

Bug: 398768875
Bug: 369155426
Test: atest SerialTests
Flag: android.hardware.serial.flags.enable_wired_serial_api
Change-Id: Iee17aaa692f7a20ed8d0f87cb48c6a8cc4377141