init.rc: Disable sysrq from the keyboard

Don't allow the accidental triggering of sysrq functionality
from the keyboard. The only expected use of sysrq functionality
is via /proc/sysrq-trigger

Please see https://www.kernel.org/doc/Documentation/sysrq.txt for
additional information on /proc/sys/kernel/sysrq

Bug: 13435961
Change-Id: I60dc92a4b2b4706e8fa34a6cead9abd449f7375f