aconfig: improve duplicate flags error message: include paths

Improve the error message returned when `aconfig dump` is fed multiple
declarations of the same flag: include the paths to the declaration
files.

In general all error messages from the protos::*::verify_* functions
should include paths to the offending files. This will be handled in a
follow-up CL.

Bug: 290300657
Test: atest aconfig.test
Test: manual: add duplicate flag and run `m all_aconfig_declarations`, inspect error message
Change-Id: I46dc23f7128dd5c68ced9f2e8518cfa89d81c2df