Cgroup APIs are guarded

The following API was added for the API level 30 and beyond. Currently,
its existence is tested using the null check which is done regardless of
the min sdk version of the compilation unit. (which in turn required us
to mark the API symbol weak regardless of the min sdk version.)

* AGroupController_getFlags

Now, we have a better way of testing the API availability;
__builtin_available. The null check is replaced with the call to the
compiler-provided macro which determines if the code is running in a
version of OS where the API is known to exist.

Bug: 150860940
Bug: 134795810
Test: m

Change-Id: I5d17fc877c553c402a62485b61af574a54658a76