Fix cgroup bpf program direction

The bpfloader loaded the cgroup bpf program in wrong place and caused
the per uid/tag stats stored in the wrong place. This patch fixed the
program loading problem and also refactored the kernel program to make
all stats record function use the same implementation. Cleaned up the
bpf instruction set that is no longer used.

Test: the per uid stats sum up is consistent with per iface stats
Bug: 30950746
Change-Id: I92c0511855f224ef98c7d75a01c99c432796d3bf