- Eliminate the explicit spongycastle dependency. All access to
primitives is done through the JCE interfaces now, which allows
us to use a secure native-backed provider like conscrypt.
- Use conscrypt for our default security provider. This gives us
fast TLS 1.2 and 1.3 support on all devices, even before they
had platform support (like 4.4).
- Update minSdk to 18. Unfortunately the JCE interfaces for GCM
primitives are JDK 7+ (!) only, which became supported by Android
at 18.
