[SECP256K1] JNI tests : remove dependency to obsolete DatatypeConverter
javax.xml.bind.DatatypeConverter is no longer part of the Java JDK
since version 9. This make it impossible to build and run the JNI test
suite with any Java > 8.
This diff replaces the 2 functions from DatatypeConverter with some
very simple ones. Performance is not really an issue here as it is test
Depends on D3834.
./contrib/teamcity/build-secp256k1.sh # Tested with Java 8 and Java 11
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3835