Switch to `libjni_legacy`

We're updating `libjni` to `0.21`. This involves several API breakages, so I'm rewriting everyone to use `libjni_legacy` first to allow `libjni` to be upgraded. Follow-up changes will update you to use `libjni`.

Change-Id: I7b151689f44d6a86ac9f03d92b6b30ed8460d543
1 file changed