Export include dirs from libsonic

Also uses cpp_std instead of cppflags.

Test: mma, identical .so files created
Change-Id: Ief58ca6c087b427c0882c2ca53539f019a3a5819
diff --git a/Android.bp b/Android.bp
index 617955b..23ca486 100644
--- a/Android.bp
+++ b/Android.bp
@@ -9,6 +9,6 @@
         "-Wall",
         "-Werror",
     ],
-
-    cppflags: ["-std=c++98"],
+    export_include_dirs: ["."],
+    cpp_std: "c++98",
 }