Refactor grouped by methods

This is a no-op refactoring, which re-write the group by methods
to use a common method that can slice NetworkStats by a given
lambda. This is useful for subsequent changes to slice NetworkStats
with different conditions.

Test: atest FrameworksNetTests:android.net.connectivity.android.net.NetworkStatsTest
Bug: 290728278
Change-Id: Ia28571021fba46fc59380798053ccdc91ddc6ae4