bootstat: Pass const string& to AddBootCompleteEvents()

This commit changes the type of `prefix` in AddBootCompleteEvents(),
which was introduced in ag/34808223, from std::string to
const std::string& to avoid an unnecessary copy.

Bug: 427910639
Test: build passes
Change-Id: Ic00dab3624aed9c77d9a190e07d2bf53225ee4e9