libprocessgroup: Proceed if activation of an optional controller fails

Not all Android kernels support all the cgroup controllers mentioned in
task_profiles.json and/or cgroups.json. Support such kernels by ignoring
certain cgroup activation failures.

Bug: 213617178
Change-Id: I90c0bd959f8a6484c4f2fbc895845e073527271e
Signed-off-by: Bart Van Assche <bvanassche@google.com>