aconfig: update aconfig_storage_read_api c++ lib

Now the read api lib no longer owns the memory mapped file. The rust lib
aconfig_storage_read_api api to get memory mapped file just returns an
Mmap rust struct. However cxx does not support Mmap data type, so we
cannot export the api to get memory mapped file from rust to c++. This
means we have to implement c++ apis to get memory mapped files.

This change is basically implementing c++ api to get memory mapped files,
mirroring the rust implementation. In addition, the integration test is
also updated to lock down the new c++ api implementation.

Bug: b/321077378
Test: atest aconfig_storage_read_api.test.cpp
Change-Id: I5e93a220cdd05cc0e5b0eb441284ac3e9dc34da7