check exported flags when building all_aconfig_declarations

This change adds extra step in the build of all_aconfig_declarations. It
will check whether all exported flags are valid. The exported flag is
valid if the flag is used for guarding API.

Test: m all_aconfig_declarations
Bug: 336800305
Change-Id: Ib1121e418f708b2ab290ed36aa3db7604cd04bdd