[DU02]Remove DevicePolicyManagerInternal from NetworkStatsAccess

NetworkStatsAccess is using DevicePolicyManagerInternal APIs, which
cannot be accessed after moving into the mainline module. So, replace
and remove those hidden API usages.

Bug: 207100745
Test: atest NetworkStatsAccessTest
Change-Id: I0802148050869d92e1f09d1d657034380029f236