aconfig: add top level flag read lib api

1, update android build soong targets as now libmemmap2 has been
introduced to android code base.

2, add three public apis for package offset query, flag offset query,
and flag value query. These apis assume the stoarge location pb file is
at /metadata/aconfig/aconfig_storage.pb. This file will be created by
storage daemon. For testing purposes, we also expose impl version of
these apis that accept a custom storage location pb file.

Bug: 321077378
Test: atest aconfig_storage_file.test
Change-Id: I91b284e2dfcdb8ff3674d463ed1c13696d9ae020