libprocessgroup: CgroupSetup should fail if a required controller fails to mount

The optional cgroup controller attribute was added in commit 7f19b1e6
("libprocessgroup: introduce optional attribute for cgroup controllers")
but has always caused CgroupSetup to return true even in the case of a
required mount failure. This function should fail in that case, only
returning true for mount failures of optional controllers.

Bug: 401031513
Change-Id: I693a91db2c58adda876f0ed6f8ea123df5a2ec61