jni: Add platform-private convenience headers

Add macros to simplify registration of both regular and @FastNative JNI
functions.

Bug: 34955272
Change-Id: If07dbd82452a28de6dcea978d46b4805d1f0eeea
diff --git a/Android.bp b/Android.bp
index bf35ebf..fe073d8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -41,7 +41,7 @@
         "-Werror",
         "-fvisibility=protected",
     ],
-    export_include_dirs: ["include"],
+    export_include_dirs: ["include", "platform_include"],
 }
 
 //