Fix a race condition in Service::Start()

The SetTaskProfiles() call modifies cgroup attributes. Modifying cgroup
attributes can only succeed after the cgroups and cgroup attributes have
been created. Hence this patch that makes the child process wait until
the parent has finished creating cgroups and activating cgroup
controllers.

Bug: 213617178
Test: Without this patch the migration to the v2 hierarchy does not work reliably. With this patch applied, the migration to the v2 hierarchy works reliably.
Change-Id: I80a7c0a35453d8fd89ed798d077086aa8ba9ea17
Signed-off-by: Bart Van Assche <bvanassche@google.com>