[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: I0430ed2c47e5a695206cb660e3971ddab6c467dd
1 file changed