Find LLNDK dumps in prebuilts/abi-dumps/vndk

LLNDK will not be finalized at the same time as APEX and NDK. The
dumps have to be placed in a different directory. The new LLNDK dumps
will be created in "vndk" directory.

Test: lunch aosp_x86_64-next-eng ; make
Bug: 314010764
Change-Id: I0726e3eabcfdbfb94a10f01ddd6dfb971552bd50