Bug 1499822 - [taskgraph] Add a tag to help differentiate 'android-stuff' from builds, r=jmaher

This will be used by |mach try chooser| to help pull 'android-stuff' tasks
(which aren't really builds) apart from the main build tasks.

Differential Revision: https://phabricator.services.mozilla.com/D14902

--HG--
extra : moz-landing-system : lando

taskcluster/ci/build/android-stuff.yml

@@ -1,4 +1,6 @@
 job-defaults:
+    tags:
+        android-stuff: "true"
     run:
         mozconfig-variant: null