Remove hidden API dependencies from StatsPullAtomService

NetworkStatsService is going to be moved into mainline module so
the hidden API usages should be removed. Thus, this patch uses
formal API to query data usage and refactor the slice networks stats
method by formal API.

Bug: 210073043
Test: build
      BytesTransferredTest and check the output of atoms
      adb shell cmd stats pull-source $ATOM_ID
Change-Id: Ie609f6c3d381f097adc30dbb9b88c0cc933e16e2