vndk-def: Rename vndk-core and vndk-ext libs.

This commit will rename vndk-core, vndk-fwk-ext, and vndk-vnd-ext
libraries so that the output will match with the post-Treble design
document.

Test: ./tests/test_vndk.py
Change-Id: I528ef380c76d71c743f1937ba94a8b72c00ce785