AppSecurity: Add traffic stats test, and fix file access test

* Fix the private file access test which would fail because the path
was wrong.
* Add a test that ensures the private file is actually "not accessible"
because it can't be as opposed to it not being there: the new test
accesses a public file created at the same time as the private file.
* Add tests around traffic stats
 . add internet permission to app that creates data.
 . generate private traffic stats (tagged sockets).
 . read back traffic stats to make sure that only public stats are visible.

Bug: 10349057
Change-Id: Ic444185bccb05cf31849ba3c986aed5d3de90303
Signed-off-by: JP Abgrall <jpa@google.com>