Add fallback in Network Tracing to old format

Perfetto has been updated to understand both the bundled and individual
packets, but the change hasn't yet hit stable. Until it does, fall back
to using the single event format.

Also fixes build warning from prior change.

Note: once the change hits Perfetto stable, the two formats behave
identically in the ui/querying/etc.

Bug: 246985031
Test: atest libnetworkstats_test
Change-Id: Ifc0e00f3c73aa1ef485f87f6ed4a9873c193ebb3
2 files changed