Make NetworkTraceHandler thread safe

This adds a mutex to guard access to all fields. The lock is taken
liberally since calls should rarely (if ever) collide (for example,
ConsumeAll is called at most every 100ms and shouldn't overlap).

Bug: 246985031
Test: atest libnetworkstats_test
Change-Id: I97791a808771bafe789091c9b54cbec0a31d1721
2 files changed