ANDROID: GKI: Enable CONFIG_NULL_TTY by default

Because GKI has various serial drivers built in (8250, pl031, samsung,
sprd, geni) for earlycon, if they succeed probes for full functionality,
(such as if they are specified in DT as UART devices), the kernel would
unconditionally set up the first console device as the default console,
which is not generally desirable for production.

Enable CONFIG_NULL_TTY and enable "console=ttynull" by default. This
causes the kernel to always be silent unless a kernel console device is
explicitly specified on the cmdline. This has no effect on earlycon
"boot console" devices, just normal "console" devices.

Bug: 223797063
Change-Id: Id4cd0615bad79462f39397427380302b1904e42b
Signed-off-by: Alistair Delva <adelva@google.com>