Add libaudioeffects header library

Add a header library for audio-effects instead of requiring modules
to use $(call include-path-for, audio-effects).

Test: m -j checkbuild
Change-Id: I21a191e7fe2911b80396ddbe84cd032592ac43b1
diff --git a/audio_effects/Android.bp b/audio_effects/Android.bp
new file mode 100644
index 0000000..9e88723
--- /dev/null
+++ b/audio_effects/Android.bp
@@ -0,0 +1,4 @@
+cc_library_headers {
+    name: "libaudioeffects",
+    export_include_dirs: ["include"],
+}