Revive ACgroupController_getFlags LLNDK function to detect missing cgroups

ACgroupController_getFlags was reverted due to LLNDK breakage, however
it allows detection of cgroups that failed to mount in a more efficient
way. Revive the function as a weakly linked symbol to allow for it to
be missing in case older LLNDK library is being used with the new
system software. This effectively reverts the commit
aa1d54f0 "Remove ACgroupController_getFlags to fix API breakage"'
except it declares ACgroupController_getFlags function as weak and
targets it for API level 30. If LLNKD library does not contain
ACgroupController_getFlags the behavior falls back to the current
way of identifying cgroups that failed to mount.

Test: build and verify correct operation with a missing cgroup
Change-Id: I9158ef53aba97972d41d71dd3396ac43796a7004
Merged-In: I9158ef53aba97972d41d71dd3396ac43796a7004
Signed-off-by: Suren Baghdasaryan <surenb@google.com>