vndk-def: Reuse cached vndk classification.

This commit replaces is_ndk with _vndk_classification so that we can
know the whether a library is in LL-NDK, SP-NDK, or HL-NDK without
repeating path matching.

Test: Run vndk_definition_tool.py on master tree.
Change-Id: Ia6894b6325f58bef74e3525f3b212a2c2776d982