Bump min SDK to 21

[bherbst]

There are a lot of reasons to bump our min SDK to 21!

First, some dependencies are bumping to 21, which will force our hand at some point anyway. Notably, OkHttp 3.13 and later requires 21. This change was done primarily to require TLS v1.2 support (more here).

While I'm not sure what our current distribution numbers are, the official distribution numbers show only 10.7% of devices globally are on <21, and the numbers are generally significantly more favorable in most of the markets where we have users (plus, our demographic skews towards more modern devices anyway).

Finally, we can leave the current version of the app available on the Play Store for some time for users who need it, but start requiring 21 for all future versions. At some point we should just drop the legacy version altogether, but that's an option if we don't want to lose those people right away.