Add permission check functions to NetworkStatsService

This is shameless copied from ConnectivityService and plan to be
used in follow-up change. For long term perspective we should
have a permission util that contains those util in one place.

Test: m -j
Change-Id: If70e6e831c0386b604d0151cc0650f68457b6e5f
Merged-In: If70e6e831c0386b604d0151cc0650f68457b6e5f
Merged-In: I837545b54f36c8f21c878cbb1fd028ff23be6cb8
Bug: 149652079
(cherry picked from commit 4da7af7e)