Convert libui to Android.bp

Along with the libinput tests, which depend on libui. For more
information on Soong, see build/soong/README.md

Change-Id: Id4fbeb114d03e37556dca59a2c9f5c9abb0a1825
diff --git a/libs/input/Android.bp b/libs/input/Android.bp
index 28df1cf..bd28af1 100644
--- a/libs/input/Android.bp
+++ b/libs/input/Android.bp
@@ -58,3 +58,5 @@
         },
     },
 }
+
+subdirs = ["tests"]