Ask netd to swap stats map before reading

To avoid protentail race problem between netd and system_server when
reading the network stats map. Always inform netd before reading the
stats and let netd to do a swap between active stats map and inactive
stats map. So the system_server can safely remove the stats after
reading.

Bug: 126620214
Test: android.app.usage.cts.NetworkUsageStatsTest
      android.net.cts.TrafficStatsTest

Change-Id: I8fa37c26bec23ffca0b29b679e72ba1189f557f1