[libnativedisplay] Cleanup map file usage

* Update Android.bp to use the map.txt file as a version script
* Move ADisplay stubs into c++ stubs, since they don't need to be
formally stable as long as they're treated as stable in practice.

Bug: 136262896
Test: builds
Change-Id: I17a1fb758e318e959538ba4bda32b886a5423d82