Fix support for optional cgroup attributes

The Linux kernel returns error code EACCES (Permission denied) if either
a cgroup attribute does not exist or if the process that is trying to
write into a cgroup attribute does not have permission to write. In
other words, it is not possible to tell the difference between these two
scenarios by checking the value of 'errno'. Hence this patch that adds a
stat() call to check whether or not a cgroup attribute exists. This
patch makes lines like the following disappear from logcat for optional
cgroup attributes:

03-22 23:16:04.816   616   649 E libprocessgroup: Failed to write '0' to /sys/fs/cgroup/./uid_10077/p
id_1695/io.group_idle: Permission denied

Bug: 213617178
Test: Booted Android in Cuttlefish and inspected logcat.
Change-Id: I6e041dfc34a52c9bdb75a8c70d99ad79b06eee06
Signed-off-by: Bart Van Assche <bvanassche@google.com>