libprocessgroup: Prevent error spam when tests disable all cpus in a cpuset

UserLifecycleTests test disables all Little cores in the course of the
test, which causes attempts to add a process into /dev/cpuset/restricted
cpuset cgroup to fail with ENOSPC error code, indicating that a process
is joining a cpuset cgroup with no online cpus. Current libprocessgroup
implementation will log an error on each such occurrence, which spams
the logs and makes it hard to analyze test results. Because this
situation does not happen in production environment (we do not offline
cpus), we can prevent flooding the logs by identifying this case,
logging an appropriate error one time and ignore all later similar errors.

Bug: 158766131
Test: adb shell "echo 0 > /sys/devices/system/cpu/cpu[0-3]/online"
Test: start some apps, observe libprocessgroup errors in the logcat
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ia91d8839d86787569c255481bde077be51c43d93
Merged-In: Ia91d8839d86787569c255481bde077be51c43d93