Inline readNetworkStatsDetailInternal, make mUseBpfStats final

This change inlines the logic from readNetworkStatsDetailInternal, and
reduces reundant checks in mUseBpfStats

Bug: 113122541
Test: atest FrameworksNetTests run, passing
Change-Id: If2ef8d8f038f32c8cf974aa02cfc1dc7e44dbad3
1 file changed