Declare the headers of the ndk_library

This creates a dependency edge between the ndk_library and its headers,
which should be a no-op in regular Soong builds. This dependency edge
will be used in the Multi-tree project to export the relevant .h files into a well
known location

The headers are in a different git project (frameworks/native)

Test: m nothing
Bug: 239044713
Change-Id: Ifd00d8d4d3771b2dbd924cea4f2ade0bafc15ed6

native/android/Android.bp

@@ -27,6 +27,9 @@ ndk_library {
     symbol_file: "libandroid.map.txt",
     first_version: "9",
     unversioned_until: "current",
+    export_header_libs: [
+        "libandroid_headers",
+    ],
 }
 
 cc_defaults {