Test packaging on android builder

Our android-nightly builder has been busted for a long time, which meant
our packaging code was not being exercised. Also, the nightly builder
only runs once a day instead of on each build, which recently allowed
a regression to slip in due to an uncaught syntax error. To prevent
future regressions, test packaging on the android builder, which runs
on try and auto builds.

buildbot/master/files/config/steps.yml

@@ -47,6 +47,7 @@ linux-rel:
 
 android:
   - ./mach build --android --dev
+  - ./mach package --android --dev
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
   - python ./etc/ci/check_dynamic_symbols.py