Fix the flaky cgroup uid bpf test

The cgroup uid traffic test is recording all the packets through the
default uid (usually root uid). If there are other networking traffic
happens when the test is running on a device, the total packets count
will be incorrect. This can be fixed by generating traffic in a socket
belong to a special test uid.

Test: bpf_test pass on device
Bug: 79488922
Change-Id: I34089336e48a0b1ffb6d5a5344dea444494c39dd