Get rid of `FileUtils.getFileDescriptorOwner`

[link2xt]

getFileDescriptorOwner is the last piece of native JNI code taken from Signal. Signal itself does not use it anymore as the last call to it was removed in https://github.com/signalapp/Signal-Android/commit/9bac88697b25f0ca4ae1104129bc5f1649579eb0 That commit is under GPLv3+ and not AGPL so we can take whatever changes are needed from there.

This getFileDescriptorOwner in jni/utils blocks moving JNI code to a separate package. Once it is done we can revive https://github.com/angelo-fuchs/dc-jni-playground and resolve https://github.com/deltachat-bot/echo/issues/9

Moving JNI code to a separate package will make it possible to upload prebuilds to Maven Central so Android developers and F-Droid will only need to run Gradle to build Delta Chat.