aconfig: use proto struct directly

Remove the hand-crafted wrappers around the structures auto-generated
from protos/aconfig.proto, and use the auto-generated structs directly
intead. This gets rid of a lot of manual repetition, and its inherent
risk.

Also unify how individual fields read from text proto are verified (e.g.
is the flag.name field a valid identifier).

Also change the intermediate cache format from JSON to binary protobuf.

The concept of a 'cache' as an intermediate internal format to represent
parsed input stays. The command line interface still refers to caches.
At the moment a cache file is identical to a parsed_file protbuf, and
the code exploits this internally.

A couple of points regarding the auto-generated structs:

  - Vectors are named in the singular (e.g. parsed_flags.parsed_flag is
    a Vec<ProtoParsedFlag>) because this improves ergonomics for all
    devs working with aconfig input files

  - The auto-generated structs have fields that are of type Option<T>
    and convenience methods (named the same as the fields) to access T

Test: atest aconfig.test aconfig.test.java
Bug: 283910447
Change-Id: I512820cc4bc6c543dea9f6a4356f863120a10be3