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
Merged-In: If2ef8d8f038f32c8cf974aa02cfc1dc7e44dbad3
Change-Id: If7d41052115ed145da8a610d676f6ed33c8d5e63
(cherry picked from commit 8c9d8c5e05cd35a340c4224c61f7fa9e95b5c861)