Make getProviderIfaceStats return Entry

This refactoring makes the method return Entry for follow-up
changes to perform arithmetic operations on them easily.
This change also removes error handling when the type is
unsupported when calling getProviderIfaceStats because it
cannot happen in practice.

Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest
Bug: N/A
Change-Id: Id26a24c80bb87c399972ef8c3abf16e0f4719cb1