Merge "Add 'ro.cpuset.default' system property for default cpusets."

`writepid <file...>`

> Write the child's pid to the given files when it forks. Meant for

  cgroup/cpuset usage.

  cgroup/cpuset usage. If no files under /dev/cpuset/ are specified, but the

  system property 'ro.cpuset.default' is set to a non-empty cpuset name (e.g.

  '/foreground'), then the pid is written to file /dev/cpuset/_cpuset\_name_/tasks.

`priority <priority>`

> Scheduling priority of the service process. This value has to be in range

        std::for_each(descriptors_.begin(), descriptors_.end(),

                      std::bind(&DescriptorInfo::CreateAndPublish, std::placeholders::_1, scon));

        // See if there were "writepid" instructions to write to files under /dev/cpuset/.

        auto cpuset_predicate = [](const std::string& path) {

            return android::base::StartsWith(path, "/dev/cpuset/");

        };

        auto iter = std::find_if(writepid_files_.begin(), writepid_files_.end(), cpuset_predicate);

        if (iter == writepid_files_.end()) {

            // There were no "writepid" instructions for cpusets, check if the system default

            // cpuset is specified to be used for the process.

            std::string default_cpuset = property_get("ro.cpuset.default");

            if (!default_cpuset.empty()) {

                // Make sure the cpuset name starts and ends with '/'.

                // A single '/' means the 'root' cpuset.

                if (default_cpuset.front() != '/') {

                    default_cpuset.insert(0, 1, '/');

                }

                if (default_cpuset.back() != '/') {

                    default_cpuset.push_back('/');

                }

                writepid_files_.push_back(

                    StringPrintf("/dev/cpuset%stasks", default_cpuset.c_str()));

            }

        }

        std::string pid_str = StringPrintf("%d", getpid());

        for (const auto& file : writepid_files_) {

            if (!WriteStringToFile(pid_str, file)) {