Move NetworkStatsExt to a standalone file

This is a no-op refactoring.

Test: atest UidAtomTests#testMobileBytesTransfer \
      UidAtomTests#testMobileBytesTransferByFgBg \
      UidAtomTests#testDataUsageBytesTransfer
Bug: 129082217
Change-Id: I52d3418c1f46acfd38bc281d87fa1d1abf6ff469
2 files changed