Add Android pushapk tasks to the promote phase in Shipit

[ahal]

My understanding is that Android builds are currently manually submitted to internal testing in the Play store. We could instead create pushapk tasks to handle this step automatically.

We had pushapk tasks defined up until recently, but I removed them because they were never working and were running in the wrong place anyway. But here's the latest version before they were removed: https://github.com/mozilla-mobile/mozilla-vpn-client/blob/4f269303f80a89cf231c1f3cf954cd38e2db969c/taskcluster/kinds/push-apk/kind.yml

It can still be used as a starting place to re-add them. These should run as part of the "promote-client" phase in shipit.