ANDROID: cpuset: Make cpusets restore on hotplug

This deliberately changes the behavior of the per-cpuset
cpus file to not be effected by hotplug. When a cpu is offlined,
it will be removed from the cpuset/cpus file. When a cpu is onlined,
if the cpuset originally requested that that cpu was part of the cpuset,
that cpu will be restored to the cpuset. The cpus files still
have to be hierachical, but the ranges no longer have to be out of
the currently online cpus, just the physically present cpus.

Bug: 174125747
Bug: 120444281
Change-Id: I22cdf33e7d312117bcefba1aeb0125e1ada289a9
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
[AmitP: Refactored original changes to align with upstream commit
        201af4c0 ("cgroup: move cgroup files under kernel/cgroup/")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[satyap@codeaurora.org: port to android-mainline kernel]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>