aconfig: split aconfig_storage_file crate

Current aconfig_storage_file crate contains two things: 1,
aconfig storage file definition (which is used by aconfig crate for
storage file creation) and 2, flag storage file read api. With this
change, we are splitting 2 into its own crate aconfig_storage_read_api

In the long run, there will be 3 crates for aconfig_storage

1, aconfig_storage_file
It contains aconfig storage file definition, as well as a binary to help
print/dump aconfig storage binary files into some readable format.

2, aconfig_storage_read_api
This crate contains flag read apis. This crate depends on
aconfig_storge_file crate. This crate will be used by codegen.

3, aconfig_storage_write_api
This crate contains flag write apis. This crate depends on
aconfig_storage_file crate. This crate will only be used by aconfig
daemon to update flag values.

Bug: b/321077378
Test: atest aconfig_storage_read_api.test; atest
aconfig_storage_file.test; atest aconfig_storage_read_api.test.rust;
atest aconfig_storage_read_api.test.cpp

Change-Id: Ia6afdc40a45c7c59652fbfd123c2b77bf6666dd9