Lower minSdkVersion to 21

This is a first look at what would be required to lower the
minSdkVersion to 21, thereby picking up devices with
Android 5.0 and 5.1 (<5% in Switzerland, approx 15% worldwide).

API level 21 has good support for Bluetooth LE. What it is missing
is some stuff related to power management, and support from the
AndroidX libraries. The solution was to back-port the
EncryptedSharedPreferences.java file from AndroidX, as it alone
did not depend on much API 23 stuff, and then adapt the key
creation stuff to do the same thing as MasterKey did, but
to fallback to another way of making the master key that works in
API 21.

This change has been tested from one Android 5.1 phone to another
Android 6 phone, and contact tracing worked.

Fixes DP-3T#16.