Use delta reads for per uid traffic stats

For the detailed network stats of each uid and tag, the native
implementation provides delta read instead of consistent stats
data since boot. Every time the readNetworkStatsDetail function gets
called, the data pulled will be cleared. We will keep a persistent data
since boot in NetworkStatsService instead.

Fixed some checkstyle complains in related files as well.

Bug: 79171384
Test: android.app.usage.cts.NetworkUsageStatsTest
      NetworkStatsServiceTest
Change-Id: I50bcf278739135081d522e50b0e88d76fd9ab131