Bump versionCode by 20

In the past, I think we bumped the version code up 20 as part of the release process. The con was that we might have forgotten. The pro was that we didn't use an unpublished versionCode and it aligned neatly. That's what I was thinking we'd do again and perhaps conveyed in the email earlier. However, since we're bumping the minSdkVersion level to something unequivocally incompatible, perhaps it makes more sense to do a major[0] and micro bump at or around the same time. At any rate, that sounds fine or better to me now than I originally supposed.

[0] http://semver.org/#summary

Searching the board for "API 15," do we want to fix any of bugs prior to completing this task?

Change 314788 had a related patch set uploaded (by Niedzielski):
WIP: Android: upgrade emulator to API 24

https://gerrit.wikimedia.org/r/314788

Change 314788 merged by jenkins-bot:
Android: upgrade emulator to API 24

https://gerrit.wikimedia.org/r/314788

Change 317851 had a related patch set uploaded (by Niedzielski):
Upgrade minimum Android version to v4.1 (API 16)

https://gerrit.wikimedia.org/r/317851

Change 317851 merged by jenkins-bot:
Upgrade minimum Android version to v4.1 (API 16)

https://gerrit.wikimedia.org/r/317851