Add separate mutex for BPF ring buffer.

This covers the ring buffer and ConsumeAll methods. This change also
removes unnecessary GUARDED_BY for TraceIfaces (making it static) and
mCallback (making it const).

This should be close to a no-op. The new lock is taken in addition to,
or instead of in the few places it's needed.

Bug: 320700806
Test: atest libnetworkstats_test
Change-Id: I45d36eba033afae1c03eac2f8d5b2ac4c9e78716
2 files changed