[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/include/apex/display.h b/libs/nativedisplay/include/apex/display.h
index 9be401e..a7eaf87 100644
--- a/libs/nativedisplay/include/apex/display.h
+++ b/libs/nativedisplay/include/apex/display.h
@@ -20,7 +20,12 @@
 #include <android/hardware_buffer.h>
 #include <inttypes.h>
 
-__BEGIN_DECLS
+// TODO: the intention of these apis is to be stable - hence they are defined in
+// an apex directory. But because they don't yet need to be stable, hold off on
+// making them stable until a Mainline module needs them.
+// __BEGIN_DECLS
+
+namespace android {
 
 /**
  * Opaque handle for a native display
@@ -130,4 +135,5 @@
  */
 int64_t ADisplayConfig_getAppVsyncOffsetNanos(ADisplayConfig* config);
 
-__END_DECLS
+} // namespace android
+// __END_DECLS