aconfig: add storage file type into storage file serialization

1, add storage file type into storage files serialization
2, update aconfig_storage_read_api integration tests to not rely on
soong to produce read only storage files. Instead, have the test process
create temp read only storage file copies.

Bug: b/312444587
Test: atest aconfig.test; atest.aconfig_storage_file.test
Change-Id: I1607fb000b48e8acffc966b16a5136578911ab3e