Allocate GID to use reserved disk space.

Filesystems like ext4 allow the setting of the "resgid" parameter
to designate a GID that is allowed to use the "reserved" disk space
(in addition to UID 0).  We'll be granting this GID to critical
system processes, so that the system is usable enough for the user
to free up disk space used by apps.

Test: builds
Bug: 36450358
Change-Id: I224bd1e597130edb411a1528872faff1ada02a89