init: create native cgroup

Cgroup feature provided by Linux kernel is one of the means used by
Android framework to control cpu usage of applications. Android
framework creates /apps and /apps/bg_non_interactive cgroups and
manages migration of tasks between those two cgroups. Tasks created
outside the scope of Android framework (kernel threads, mpdecision
etc) are however left in "root" cgroup, where each task is considered
to be a cgroup of its own, thus giving each such task more CPU
resource than desired. Such tasks have been found to interfere with
smooth functioning of Android application and have been responsible
for effects such as increased janks or bad user-experience.

Fix this problem by creating a /native cgroup to encapsulate all tasks
at bootup. /native cgroup is assigned shares of 1024 and real-time
bandwidth of 15% (150ms out of every 1sec). The real-time bandwidth is
unchanged (from what currently exists) while the combined shares of
1024 for all "native" tasks will help curb interference between native
and Android tasks.

Change-Id: I988b6145ab7cfbd94cbbab4d63fadc3bab6551fa
Merge "init: create native cgroup"