Reduce minSdkVersion to 21

[jeffallen]

I tried to be a beta tester, but my phone is Android 5.1 (API level 21) and thus I cannot run the app.

Here is an analysis of what should still be possible even in older phones, from the tracing app being developed by colleagues of mine at EPFL: DP-3T/dp3t-sdk-android#16

[ivnsch]

Thanks for the suggestion! Yeah, the team wants to support older versions, to reach as much users as possible. If you want to work on this, it would be welcome.

The TCN library (which handles BLE and crypto) would have to be adapted too (currently min is 23). I'm not sure about the effort involved. It may be easy.