Check for /dev/memcg/system/tasks before writing to it

Fixes the following errors in the logs on devices without /dev/memcg
mounted:

09-08 10:07:22.991   732   732 E Zygote  : couldn't write 1124 to
/dev/memcg/system/tasks
09-08 10:07:22.986   732   732 W main    : type=1400 audit(0.0:8):
avc: denied { create } for name="tasks" scontext=u:r:zygote:s0
tcontext=u:object_r:cgroup:s0 tclass=file permissive=0

Bug: 67860826
Test: boot Taimen, no more error/denial in the logs.
Change-Id: I37f481fa4e9e9116688cca4f090aa26f96f602c9