aconfig: add a storage file read api to get file version number

1, Add unit tests in aconfig_storage_file crate to lock down that the
first four bytes of serialized storage files must be the version number.

2, Add a storage file read api to read version number directly from a
file without mmapping the file. This api is need by aconfig storage
daemon.

3, Update integration tests (rust and c++) to include this api.

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

Change-Id: I432893d43895ad5022fcb614aa14de89acd9d220