aconfig: improve flag value tracing

Improve how the flag values (state, permission) are tracked when parsing
config and override files: migrate from a raw string to a proper struct,
as this will make it easier when aconfig will output this data in some
structured format for other tools to consume.

Also, rename Cache.debug to Cache.trace.

Bug: 279485059
Test: atest aconfig.test
Change-Id: Idad57c969515f697e9065429d8a44c38d8a512d2