vndk-def: Support Treble VNDK directory layout

This commit adds /system/lib/vndk and /vendor/lib/vndk to the search
path for vendor modules.

Test: Run `vndk_definition_tool.py vndk` on the device with Treble VNDK
directory layout.
Test: Run `vndk_definition_tool.py vndk` on sailfish and nothing
changes (including warning/error messages).

Change-Id: I7499691c05d66213441371bd18d5b84cf79d6221