adb: grant it AID_NET_BW_STATS for full /proc/net/xt_qtaguid/stats

Normally only bandwidth stats for the uid reading .../stats is returned.
With the extra group, adb shell will be able to read stats from
all UIDs.
This is to be used to offer data usage profiling to developers.

Change-Id: If3d2941ce5aa4dbb1a23947b97b893149ba224f7