aconfig: add 'verbose' dump format

Add a new --format=verbose dump output format. This is identical to the
default 'text' format but includes the source files from which the flag
declaration and values were read, and is intended to help debug why a
flag isn't set to some expected value.

Bug: 283910447
Test: atest aconfig.test
Test: printflags --format=verbose # manually inspect output
Change-Id: I03a1c8d940e7a0c6f91e986c5bafa4aa6cd1125a