aconfig: add aconfig_storage_metadata proto

Introduce a new proto to capture storage file location for each
container. This proto file will appeara as
/metadata/aconfig/storage_file_location.pb. Storage service daemon is
responsible for writing entires to it when a new storage file set is
available. The flag read lib will use this file to find the
corresponding storage file and mmap them.

Bug: b/321077378
Test: atest aconfig_storage_file.test
Change-Id: I226e76be895805dce52a075050dcd5b42d337be8