ART: Move JniConstants::init from ART to libcore

This should live with libcore. There's no deep magic in the runtime
startup.

Also remove dependency on libnativehelper shared library.

Bug: 65522645
Test: m
Test: m test-art-host
Test: Device boots (bullhead)
Change-Id: I6d03144fa02461d31e9e26c27cdb3a9a9510122d