Package media/*/*.h headers as a headers lib: media_plugin_headers

Headers under frameworks/native/include/media serves as an interface
that is meant to be implemented by vendors. For example.
media/hardware/CryptoAPI.h is implemented under
vendor/widevine/libwvdrmengine/mediacrypto.

When building with BOARD_VNDK_VERSION, the headers are not accessible to
vendors since they have been included via the global include paths and
the paths are not available when building with BOARD_VNDK_VERSION.

So, we need to make a "headers library" for the media headers and modify
vendors to use the headers lib. Usually, we have been doing this by
moving the headers into a subdirectory (usually 'include') of a module
that implements the headers. But, this approach can't be used at this
time since the media headers are implemented by many modules.

The chosen solution is to a separate directory headers/media_plugin and
define a headers lib named media_plugin_headers there.

Note: frameworks/native/headers will now be the home for such
header-only libraries.

Bug: 63120269
Test: build
Change-Id: I67d625706b40d06c8f0602284fbcc1cc8b174c6f
diff --git a/Android.bp b/Android.bp
index cd05b21..9757797 100644
--- a/Android.bp
+++ b/Android.bp
@@ -8,6 +8,7 @@
 
 subdirs = [
     "cmds/*",
+    "headers",
     "libs/*",
     "opengl",
     "services/*",