Block access to xt_qtaguid proc files

In the next Android release, there will be devices that have no
xt_qtaguid module at all and framework and netd will decide which code
path it takes for trafficStats depending on the device setup. So all
apps and services should not depend on this device specific
implementation anymore and use public API for the data they need.

Bug: 114475331
Bug: 79938294
Test: QtaguidPermissionTest

Change-Id: I0d37b2df23782eefa2e8977c6cdbf9210db3e0d2