PackageParser2: collect certificates if requested

This is a relatively easy and safe change that should significantly
reduce boot time.

Test: atest google/perf/boottime/boottime-test
Test: atest PackageManagerTest
Bug: 155535721
Bug: 155513789
Bug: 155525390
Change-Id: Ib5152892184d407361ce3698575075ec0138edbf