Permissions to protect bandwidth statistics.

Introduces new "net_bw_stats" group which will protect reading
detailed bandwidth statistics from the kernel.  Also introduce
"net_bw_acct" group which will enable specific applications to
request that their network traffic be counted against other UIDs.

This change associates manifest permissions with the low-level GIDs.

Change-Id: If3fc28053afda201ff305d798a8878ff1f35b997