Add support for cgroup controller flag bitmask in libcgrouprc

Add ACgroupController_getFlags to retrieve flags for a cgroup controller.
Currently the only supported flag is CGROUPRC_CONTROLLER_FLAG_MOUNTED
(0x01) to distinguish between controllers that were able to mount and the
ones that failed due to missing kernel support.

Bug: 124080437
Test: libcutils_test with cpuset and schedtune controllers disabled
Change-Id: I42f8a81624e7b056b6a4a369e61b40d8b649b21b
Signed-off-by: Suren Baghdasaryan <surenb@google.com>