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

rootdir/init.rc

@@ -13,6 +13,9 @@ on early-init
     # Set init and its forked children's oom_adj.
     write /proc/1/oom_score_adj -1000
 
+    # Disable sysrq from keyboard
+    write /proc/sys/kernel/sysrq 0
+
     # Set the security context of /adb_keys if present.
     restorecon /adb_keys