Add a header lib for libarect

This allows removing some actual link dependencies from things
that really only needed the arect.h header (e.g. libRScpp.so was
linking with libgui, but didn't need to).

Test: build
Change-Id: I3d95b9901333744bab3fd3e28bd00e87d536d769
diff --git a/libs/arect/Android.bp b/libs/arect/Android.bp
index ad8287c..2518b14 100644
--- a/libs/arect/Android.bp
+++ b/libs/arect/Android.bp
@@ -13,13 +13,18 @@
 // limitations under the License.
 
 ndk_headers {
-    name: "libarect_headers",
+    name: "libarect_headers_for_ndk",
     from: "include/android",
     to: "android",
     srcs: ["include/android/*.h"],
     license: "NOTICE",
 }
 
+cc_library_headers {
+    name: "libarect_headers",
+    export_include_dirs: ["include"],
+}
+
 cc_library_static {
     name: "libarect",
     host_supported: true,