Move JniConstants to points of use

Removes the JniConstants from the libnativehelper API.

Most of the constants are moved into libcore.

Light tidying up in touched files.

Bug: 119840313
Test: atest libnativehelper/test
Test: atest CtsLibcoreTestCases
Test: art/test.py --host -g -j16
Test: device boots

Change-Id: I4c0daab450d2efd7e6bd7705e27dbb6309fcb92d