[SECP256K1] JNI tests : remove dependency to obsolete DatatypeConverter

Summary:
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
only code.

Depends on D3834.

Test Plan:

./contrib/teamcity/build-secp256k1.sh # Tested with Java 8 and Java 11

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3835