Fix IsUsable to check for each controller separately

CgroupController::IsUsable is using a global static variable to store the
existence of the controller. That means the first controller existence
check would affect all other controllers. Fix this by making this variable
to be a member of CgroupController class so that each controller can check
for its existence independently of other controllers.

Fixes: aa1d54f0 ("Remove ACgroupController_getFlags to fix API breakage")
Bug: 136020193
Test: adb shell cat /proc/$pid/task/*/cgroup" prints "cpuset:/top-app"
Test: for new launched activity process
Change-Id: I4741a9126ea494122d5b2b1a0c4d7252bff6025c
Signed-off-by: Suren Baghdasaryan <surenb@google.com>