Add r8/proguard rules for JNA

[chenxiaolong]

JNA's Android .aar artifact doesn't include any r8/proguard rules. According to their FAQ [1], application developers are expected to copy and paste the 4 rules into their proguard configs. Without these rules, SodiumAndroid() initialization would fail because JNA's initialization can't find the fields it needed via reflection with r8 renaming fields to single letter names.

java.lang.UnsatisfiedLinkError: Can't obtain peer field ID for class com.sun.jna.Pointer

This only affected release builds since debug builds don't have r8 enabled.

[1] https://github.com/java-native-access/jna/blob/master/www/FrequentlyAskedQuestions.md#jna-on-android

Fixes: #357

[chenxiaolong]

In my release build, the peer field in com.sun.jna.Pointer got renamed to b by r8, so this:

https://github.com/java-native-access/jna/blob/5.19.1/native/dispatch.c#L2872-L2875

couldn't find the field by name.

app/build/outputs/mapping/standardRelease/mapping.txt:

com.sun.jna.Pointer -> com.sun.jna.Pointer:
# {"id":"sourceFile","fileName":"Pointer.java"}
    long peer -> b

EDIT: JNA should really be providing a proguard config in their .aar artifacts since they already have Android-specific builds. I wonder why they don't. Most Android libraries that require special configs will include that.

[tmo1]

Thanks much! I tested extensively before comitting and releasing this - but only with debug builds :|

[tmo1]

To be fair to the LazySodium project, it does recommend adding these ProGuard rules in its own documentation, but I had missed it.