Deduplicate items after clear interface in NetworkStats

Follow-up from commit Ie60829a65d0d9b5b63ad353695a820c0586e3665,
the interface field was cleared before returning the result to
the caller. However, this can cause problems in the
NetworkStats#subtract method. If the interface field is cleared,
the findIndexHinted method can match to a wrong entry.
This is because the keys of multiple entries will now be the
same. This can cause the subtract result to be unexpectedly
large and the return value of getUidStatsForTransport to be
mismatched with the values retrieved from other APIs.

Test: atest FrameworksNetTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest \
      FrameworksNetTests:android.net.connectivity.android.net.NetworkStatsTest
Bug: 290728278
Change-Id: I891ab29b8a2902663febc7c32b04417caf510926