support pushapk in chain of trust

[escapewindow]

@JohanLorenzo This adds initial pushapk support to the chain of trust verification in scriptworker.

Speaking of: we're ready to turn on tier1 taskcluster android nightlies tonight, and that will possibly merge to aurora next week. If that happens, we would be able to then get pushapk into the nightly graph with chain of trust support, and we could turn off the pushapk scheduler.

If that doesn't happen next week, we're still thinking about getting tc android nightlies turned on in Aurora over the next 6-7 weeks, so it'll happen sooner or later.

[coveralls]

Coverage remained the same at 100.0% when pulling 2ac0e18 on escapewindow:pushapk into ccb4f52 on mozilla-releng:master.

[JohanLorenzo]

Just checking the task was run on a scriptworker instance sounds good to me. I'll update the configs with the new "pushpk" type

[coveralls]

Coverage remained the same at 100.0% when pulling abec688 on escapewindow:pushapk into d9355eb on mozilla-releng:master.