[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/Android.bp b/libs/nativedisplay/Android.bp
index d37b9a1..bcc99fc 100644
--- a/libs/nativedisplay/Android.bp
+++ b/libs/nativedisplay/Android.bp
@@ -40,6 +40,8 @@
         "-Wno-unused-function",
     ],
 
+    version_script: "libnativedisplay.map.txt",
+
     srcs: [
         "AChoreographer.cpp",
         "ADisplay.cpp",