Changes for jni 0.20.0

[simlay]

Over in #14, I've found that some of the android ndk crates are using jni 0.20. Here are some of those changes:

• The From trait implementations converting jni_sys types like jobject to JObject have been replaced with unsafe ::from_raw
• The lifetime of AutoArray is no longer tied to the lifetime of a particular JNIEnv reference.

I'd like to update to 0.21.0 evenutally but when I tried that, I ended up with 44 errors rather than 3 so let's start small.

Some of the changes are because my vim setup removes white space at the end of lines. I can remove those changes if it pleases you.

When I run just test locally, it passes and works.

[bluejekyll]

This looks good. Thanks for the PR!

[simlay]

@bluejekyll how might I get CI to run? It looks like it was disabled due to inactivity. I'm guessing this is because it was on a cron schedule but there's not been much activity in a while. You might need to push a button but I'm not sure.