[SECP256K1] Create a different library when building with JNI

Summary:
The current behavior is to build a secp256k1 library that has the JNI
binding feature if enabled.
This diff keeps the main secp256k1 library intact, and creates another
secp256k1_jni library with JNI support in it. The java test is updated
to reflect this.

Depends on D4053.

Test Plan:

cd secp256k1
./autogen.sh
mkdir build && cd build
../configure --enable-jni --enable-experimental --enable-module-ecdh
make

Check there is a libsecp256k1_jni.so file under .libs/

make check-java

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4058