aconfig: sort items in cache by name

Introduce a builder pattern for constructing a cache from flag
declarations and flag values. Teach the builder to sort the flags by
name as the last step. This will ensure consistent dump output
regardless of the order flags are specified in the input files.

Bug: 279485059
Test: atest aconfig.test
Change-Id: Icdd62f51fa3761a469663f17581a83d9909e9ffe