Fix Unicode handling in Java bindings

[VivekPanyam]

Summary:

This PR fixes how the Java bindings deal with Unicode.

Java's Unicode handling methods in JNI (e.g. GetStringUTFChars, NewStringUTF) deal with "modified UTF-8" encoding.

According to most descriptions I found online, UTF-8 and modified UTF-8 are quite similar with one of the main differences being how they deal with embedded null bytes:

• https://docs.oracle.com/javase/6/docs/api/java/io/DataInput.html#modified-utf-8
• https://en.wikipedia.org/wiki/UTF-8#Modified_UTF-8
• https://stackoverflow.com/questions/7921016/what-does-it-mean-to-say-java-modified-utf-8-encoding

Unfortunately, this does not mean that UTF-8 and modified UTF-8 are interoperable in cases without embedded null bytes. In fact, the encodings can be quite different and have different lengths.

This causes problems when doing something like this:

Java -> Neuropod Java bindings -> C++ -> Neuropod Python Model

because the JNI string methods in the Java bindings will encode strings in "modified UTF-8" and Numpy on the python side will try to decode them as UTF-8.

To fix this issue, I changed the Java bindings to explicitly do a UTF-8 conversion in C++.

Java internally represents strings in UTF-16 so the bindings now get that representation and explicitly convert to UTF-8 (and do the reverse when going from C++ to Java).

Test Plan:

• Changed the Java string model tests to include Unicode characters
• Added a test to run a Python Neuropod model from Java (failed before implementing the fix, succeeded after the fix)

[codecov]

Codecov Report

Merging #533 (cde3f5c) into master (c4ed8c3) will increase coverage by 0.00%.
The diff coverage is 90.47%.

@@           Coverage Diff           @@
##           master     #533   +/-   ##
=======================================
  Coverage   86.58%   86.58%           
=======================================
  Files         104      104           
  Lines        6940     6955   +15     
=======================================
+ Hits         6009     6022   +13     
- Misses        931      933    +2     

Impacted Files	Coverage Δ
...rce/neuropod/bindings/java/src/main/native/utils.h	100.00% <ø> (ø)
...thon_bridge/_neuropod_native_bootstrap/executor.py	90.14% <50.00%> (-2.51%)	⬇️
...rc/main/native/com_uber_neuropod_NeuropodTensor.cc	72.26% <100.00%> (ø)
...ce/neuropod/bindings/java/src/main/native/utils.cc	72.30% <100.00%> (+6.26%)	⬆️

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