Audit app access to /proc/net/*

Many processes including third party apps are expected to
access /proc/net/xt_qtaguid/stats. Give this file a new label
to avoid spamming the logs and temporarily grant read access to
all processes.

Read-only permission is adequate for all processes based on unix
permissions.
sailfish:/ # ls -laZ /proc/net/xt_qtaguid/stats
-r--r--r--  1 root net_bw_stats u:object_r:proc_net_xt_qtaguid_stats:s0 stats

Bug: 9496886
Bug: 68016944
Bug: 70722355
Test: Build/flash Sailfish. Browse in Chrome and watch videos in youtube.
    No "denied" or "granted" selinux messages observed in the logs.

Change-Id: I29f1ee806c8149988b9b93a950790d14754927ef