aconfig: create aconfig_storage_file crate

Move aconfig storage file definition into its own crate, as flag read
lib will need it as well. The new crate is placed under
aconfig_storage_file dir.

Bug: b/321077378
Test: atest aconfig.test, Cargo test at repo root level
Change-Id: I91e8ada2c9bab479ed81cf03824042f3fa460be5