Use a version script for libaconfig_storage_read_api_cc

Restrict the set of exported symbols to those in the aconfig_storage
namespace by way of a version script. This shrinks the shared lib size
by ~75%, from ~800KB to <200KB.

Bug: 336657207
Test: m
Change-Id: I56044fe667a713cf1d94f96c992f379a5725850f