BpfHandler: abort on U+ if unexpected cgroup mount path

This is defined here:
http://cs/h/android/platform/superproject/+/master:system/core/libprocessgroup/profiles/cgroups.json?l=27
and has been a constant since
  https://android-review.googlesource.com/c/platform/system/core/+/1324649
was merged back on August 21, 2020 (ie. Android S)

But currently there's no easy way to get at this constant
from mainline code... which means it's very difficult
to do any bpf cgroup attach/detach from mainline.

btw. this appears to also be already hardcoded in:
http://cs/h/android/platform/superproject/+/master:packages/modules/Connectivity/service/src/com/android/server/connectivity/ConnectivityNativeService.java?l=48


as well

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I99f4a5a26dd10f6ea70bf73114af0d18098de630