Add tools to subdirs

because once the Android.bp finder is enabled, it will be found anyway,
and this will allow us to confirm separately that adding this Android.bp
is successful.

Bug: 64363847
Test: m -j
Change-Id: Id4593099cdb3d839054700cc198f6c808c3c6642
diff --git a/libs/ui/Android.bp b/libs/ui/Android.bp
index cf79762..07aba32 100644
--- a/libs/ui/Android.bp
+++ b/libs/ui/Android.bp
@@ -117,4 +117,7 @@
     vendor_available: true,
 }
 
-subdirs = ["tests"]
+subdirs = [
+    "tests",
+    "tools",
+]