hiddenapi: Fix map insertion bug

`hiddenapi` would use std::map::insert to overwrite an entry, which
is wrong and will not overwrite the entry. This caused all entries
to be classified as private. Fix the bug.

Bug: 79409988
Test: m appcompat, check hiddenapi-public-list.txt not empty
Change-Id: Ie1c1fad2dfff9ff0332b5196d605d512d6dedc15