Shift sysctl.d writing code to a new script

sysctl-set-value also sets the value on the build system, which
may cause unintended side effects while building images on
machines. Since most of these side effects are harmless, most
call sites can continue to use it, but also allow elements to
set a sysctl value that does not take immediate effect on the
host by just writing the file.

Change-Id: Ife8a156a22a305603c6b0af1c095e073e492a4b9
Merge "Shift sysctl.d writing code to a new script"