SpongyCastle is a fork of BouncyCastle. We originally used this fork to 1) have
access to scrypt and 2) prevent a package name collision with the bundled
BouncyCastle. We don't actually need to use the fork anymore, because the
package name of the bundled BouncyCastle was changed in Android. SpongyCastle
has also gotten quite outdated in recent years.
The built-in version of BouncyCastle is replaced with the one bundled with the
app at runtime, so that we have a recent version even on older Android versions.
This also updates Gradle and the Gradle Android plugin, to fix a build error I
was running into when I added the dependency to BouncyCastle.
