libprocessgroup: fall back to cpuset in get_sched_policy

Since vendor has a way to override the group cpu/schedtune setup, we
cannot assume the group will always return valid data. This CL let
get_sched_policy to fallback to cpuset if no valid data found in
cpu/schedtune cgroup. In longer term, we should find a way to cache the
group or app's process state in framework other than relying on reading
cgroup back.

Test: /data/nativetest64/libcutils_test/libcutils_test
Bug: 210066228
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: I8b4396365a7fc2d93e3a22746195585c140eef3c