Check for bpf support at NetworkStatsService start

The current implementation check for bpf map existance whenever a
NetworkStats request comes in. The check is not efficient and may
require additional permission for threads. So a member variable is added
to NetworkStatsService to store the bpf configuration status and is
initialized when NetworkStatsService start.

Test: -m CtsNetTestCases -t android.net.cts.TrafficStatsTest
      -m CtsUsageStatsTestCases -t android.app.usage.cts.NetworkUsageStatsTest
Bug: 79994577

Change-Id: I872ae18fb3e9631a4e6c4d773fefbba32e60a03b