Accumulate NetworkStats since boot in StatsPullAtomService

BytesTransfer atoms need to return stats since boot, whilst
NetworkStats persists data only for a limited amount of time.

It is necessary to accumulate stats internally to return
correct data after time since boot exceeds persistence window.

Bug: 352537247
Test: atest FrameworksServicesTests:NetworkStatsAccumulatorTest
Flag: com.android.server.stats.accumulate_network_stats_since_boot
Change-Id: Icdd641dba8d359ae5e054287af042a23082a896f