aconfig_storage_read_api: make map_file function public.

With aconfigd moving to rust implementation. It not only needs to open
boot copy, but also persist and default copy. The get_mapped_file
function is meant to be used by codegen which only opens boot copy. Thus
need to make map_file function public, just like what we did in c++.

Bug: b/321077378
Test: atest
Change-Id: I4fd2a5e0cf7839fde43e83b8888488caa74fb1b9