Update nullability annotation for NetworkStats atom queries

The previous annotation incorrectly implied that
getUidNetworkStatsSnapshotForTemplateLocked can return null.

Bug: 352537247
Test: atest FrameworksServicesTests:NetworkStatsAccumulatorTest
Test: atest android.cts.statsdatom.net.BytesTransferredTest
Flag: EXEMPT refactor
Change-Id: I43cc466f809bb19cdfd0ee880bcab027b82f3b2c