logd: allow logd to write to /dev/cpuset files

Required by logd on devices with USE_CPUSETS defined.

Make /dev/cpuset/background, /dev/cpuset/foreground and
/dev/cpuset/task writeable by system gid. Add logd to system
group for writing to cpuset files and to root group to avoid
regressions. When dropping privs, also drop supplementary groups.

Bug: 22699101
Change-Id: Icc01769b18b5e1f1649623da8325a8bfabc3a3f0