Upgrade Spongy Castle and some other dependencies

This branch updates Spongy Castle to the latest release, 1.56, which changes the behaviour of Poly1305 to match the NaCl implementation, so we no longer need a workaround.

A few other dependencies have also been updated, where it was possible to do so without breaking things. The jars for weupnp and JNA have been replaced with Gradle dependencies (see #301).

The new version of Material Tap Target Prompt allows the opacity of the target to be reduced, which might help with #924.