[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
diff --git a/libs/nativedisplay/ADisplay.cpp b/libs/nativedisplay/ADisplay.cpp
index 6566538..1e25049 100644
--- a/libs/nativedisplay/ADisplay.cpp
+++ b/libs/nativedisplay/ADisplay.cpp
@@ -129,6 +129,8 @@
 }
 } // namespace
 
+namespace android {
+
 int ADisplay_acquirePhysicalDisplays(ADisplay*** outDisplays) {
     const std::vector<PhysicalDisplayId> ids = SurfaceComposerClient::getPhysicalDisplayIds();
     const size_t size = ids.size();
@@ -298,3 +300,5 @@
 
     return reinterpret_cast<DisplayConfigImpl*>(config)->appOffset;
 }
+
+} // namespace android