Restrict TrafficStats due to privacy concerns.

Granular per-UID network statistics can be used to infer user
behavior over time, so they fall under the umbrella of the
PACKAGE_USAGE_STATS permission.

Since we can't check app-ops based permissions in the kernel, the
best we can do is redirect users to the NetworkStatsManager class,
which offers a much more robust historical data set.

Bug: 27577101
Change-Id: I696bdc5e0b3d7e24acf35f388d0ab13617ed8af3