aconfig: test updates

Previously, three seperate crates create their own test baseline which
are nearly identical. This is inconvinent when updating the baselines.
All three locations needs to be touched. This cl merges functions to
create test storage file base struct into aconfig_storage_file crate

Bug: b/321077378
Test: atest aconfig.test
Change-Id: Ic97437f856763347a11de8b449592292014bcce3