Add LogEvent::hasAttributionChain()

Optionally, accept a std::pair<int, int>* that is populated with the
starting and ending indices of attribution nodes in FieldValues vector.

Bug: 154286011
Test: bit statsd_test:*
Change-Id: I6f19abea7fbb27712b6c2d5a7679aa4f428c191d
6 files changed