Add libapexutil static library

This provides a helper function to get active APEX packages by scanning
/apex directory. Even though this feature is already provided by
ApexService::getActivePackages(), but some processes (e.g.
linkerconfig) which can't rely on IPC may use this library.

Bug: 144659031
Test: atest --host libapexutil_tests
Change-Id: I92c69637e0599c1db05a323aaabb4c30ca135f8f
4 files changed