aconfig: dump: support multiple caches

Teach `aconfig dump` to read multiple caches at the same time.

A cache represents a single namespace, and the Android build creates
multiple caches for different namespaces. By passing in all those cache
files to `aconfig dump`, aconfig will create a complete overview of all
flags in the Android tree.

Caches are traversed in order of namespace to produce the same output
regardless of the order the cache files are given on the command line.
If two caches use the same namespace, their order with respect to each
other is undefined.

Bug: 279485059
Test: atest aconfig.test
Change-Id: I54c3950bbb7b2be7d96c8928e78ae83aa626c2e2