Account for interpolation loss in cumulative network stats

This is a copy of ag/30155473, which got mistakenly reverted
in ag/30195550 due to unrelated SysUI test failures.

Bug: 352537247
Test: atest FrameworksServicesTests:NetworkStatsAccumulatorTest
Flag: com.android.server.stats.accumulate_network_stats_since_boot
Change-Id: Ic8289db19bcd8a6a13e3fd15e5ba49e628809c05