Expose libstatssocket as a stable C API

We create a .map.txt file that lists all the stable entry points into
libstatssocket. This should allow other APEXes to link to libstatssocket
without having to copy the library within the APEX.

Test: m -j libstatssocket
Bug: 146377784
Change-Id: I9f77a0c380b6884d9ca60807a8974380420cfe0a