Export libgui headers from media modules

libstagefright, libmedia_ndk and libmedia_omx

Also, fix the reference to HGraphicBufferProducer in
AImageReaderWindowHandleTest.

Test: Builds

Bug: 112508112
Change-Id: I2ce0e4fd395a0c733aaf98b7397d7183c7475672
diff --git a/media/ndk/Android.bp b/media/ndk/Android.bp
index f9f1acc..f4cc704 100644
--- a/media/ndk/Android.bp
+++ b/media/ndk/Android.bp
@@ -101,6 +101,10 @@
 
     export_include_dirs: ["include"],
 
+    export_shared_lib_headers: [
+        "libgui",
+    ],
+
     product_variables: {
         pdk: {
             enabled: false,